Primitives: mention MeshTools::owned() in immutable primitive docs.

src/Magnum/Primitives/Axis.h

@@ -41,7 +41,8 @@ Two color-coded arrows for visualizing orientation (XY is RG), going from
 @cpp 0.0f @ce to @cpp 1.0f @ce on the X and Y axis. @ref MeshPrimitive::Lines
 with @ref MeshIndexType::UnsignedShort indices, interleaved
 @ref VertexFormat::Vector2 positions and @ref VertexFormat::Vector3 colors. The
-returned instance references data stored in constant memory.
+returned instance references data stored in constant memory --- pass the data
+through @ref MeshTools::owned() to get a mutable copy, if needed.
 
 @image html primitives-axis2d.png width=256px
 
@@ -56,7 +57,8 @@ Three color-coded arrows for visualizing orientation (XYZ is RGB), going from
 @cpp 0.0f @ce to @cpp 1.0f @ce on the X, Y and Z axis.
 @ref MeshPrimitive::Lines with @ref MeshIndexType::UnsignedShort indices,
 interleaved @ref VertexFormat::Vector3 positions and @ref VertexFormat::Vector3
-colors. The returned instance references data stored in constant memory.
+colors. The returned instance references data stored in constant memory ---
+pass the data through @ref MeshTools::owned() to get a mutable copy, if needed.
 
 @image html primitives-axis3d.png width=256px
 

src/Magnum/Primitives/Crosshair.h

@@ -39,7 +39,8 @@ namespace Magnum { namespace Primitives {
 
 2x2 crosshair (two crossed lines), centered at origin. Non-indexed
 @ref MeshPrimitive::Lines with @ref VertexFormat::Vector2 positions. The
-returned instance references data astored in constant memory.
+returned instance references data astored in constant memory --- pass the data
+through @ref MeshTools::owned() to get a mutable copy, if needed.
 
 @image html primitives-crosshair2d.png width=256px
 
@@ -52,7 +53,8 @@ MAGNUM_PRIMITIVES_EXPORT Trade::MeshData crosshair2D();
 
 2x2x2 crosshair (three crossed lines), centered at origin. Non-indexed
 @ref MeshPrimitive::Lines with @ref VertexFormat::Vector3 positions. The
-returned instance references data stored in constant memory.
+returned instance references data stored in constant memory --- pass the data
+through @ref MeshTools::owned() to get a mutable copy, if needed.
 
 @image html primitives-crosshair3d.png width=256px
 

src/Magnum/Primitives/Cube.h

@@ -40,7 +40,8 @@ namespace Magnum { namespace Primitives {
 2x2x2 cube, centered at origin. @ref MeshPrimitive::Triangles with
 @ref MeshIndexType::UnsignedShort indices, interleaved
 @ref VertexFormat::Vector3 positions and flat @ref VertexFormat::Vector3
-normals. The returned instance references data stored in constant memory.
+normals. The returned instance references data stored in constant memory ---
+pass the data through @ref MeshTools::owned() to get a mutable copy, if needed.
 
 @image html primitives-cubesolid.png width=256px
 
@@ -52,9 +53,10 @@ MAGNUM_PRIMITIVES_EXPORT Trade::MeshData cubeSolid();
 @brief Solid 3D cube as a single strip
 
 2x2x2 cube, centered at origin. Non-indexed @ref MeshPrimitive::TriangleStrip
-with @ref VertexFormat::Vector3 positions. The returned instance references
-data stored in constant memory. No normals or anything else --- use
-@ref cubeSolid() instead if you need these.
+with @ref VertexFormat::Vector3 positions. No normals or anything else, use
+@ref cubeSolid() instead if you need these. The returned instance references
+data stored in constant memory --- pass the data through @ref MeshTools::owned()
+to get a mutable copy, if needed.
 
 Vertex positions of this mesh can be also generated directly in the vertex
 shader using @glsl gl_VertexID @ce ([source](https://twitter.com/turanszkij/status/1141638406956617730),
@@ -71,7 +73,8 @@ MAGNUM_PRIMITIVES_EXPORT Trade::MeshData cubeSolidStrip();
 
 2x2x2 cube, centered at origin. @ref MeshPrimitive::Lines with
 @ref MeshIndexType::UnsignedShort indices and @ref VertexFormat::Vector3
-positions. The returned instance references data stored in constant memory.
+positions. The returned instance references data stored in constant memory ---
+pass the data through @ref MeshTools::owned() to get a mutable copy, if needed.
 
 @image html primitives-cubewireframe.png width=256px
 

src/Magnum/Primitives/Plane.h

@@ -89,7 +89,8 @@ enum class CORRADE_DEPRECATED_ENUM("use PlaneFlags instead") PlaneTextureCoords:
 @ref VertexFormat::Vector3 normals in positive Z direction, optional
 @ref VertexFormat::Vector4 tangents and optional @ref VertexFormat::Vector2
 texture coordinates. The returned instance may reference data stored in
-constant memory.
+constant memory --- pass the data through @ref MeshTools::owned() to get a
+mutable copy, if needed.
 
 @image html primitives-planesolid.png width=256px
 
@@ -118,7 +119,8 @@ CORRADE_IGNORE_DEPRECATED_POP
 
 2x2 square on the XY plane, centered at origin. Non-indexed
 @ref MeshPrimitive::LineLoop on the XY plane with @ref VertexFormat::Vector3
-positions. The returned instance references data stored in constant memory.
+positions. The returned instance references data stored in constant memory ---
+pass the data through @ref MeshTools::owned() to get a mutable copy, if needed.
 
 @image html primitives-planewireframe.png width=256px
 

src/Magnum/Primitives/Square.h

@@ -79,7 +79,8 @@ enum class CORRADE_DEPRECATED_ENUM("use SquareFlags instead") SquareTextureCoord
 2x2 square, centered at origin. Non-indexed @ref MeshPrimitive::TriangleStrip
 with interleaved @ref VertexFormat::Vector2 positions and optional
 @ref VertexFormat::Vector2 texture coordinates. The returned instance
-references data stored in constant memory.
+references data stored in constant memory --- pass the data through
+@ref MeshTools::owned() to get a mutable copy, if needed.
 
 @image html primitives-squaresolid.png width=256px
 
@@ -103,7 +104,8 @@ CORRADE_IGNORE_DEPRECATED_POP
 
 2x2 square, centered at origin. Non-indexed @ref MeshPrimitive::LineLoop with
 @ref VertexFormat::Vector2 positions. The returned instance references data
-stored in constant memory.
+stored in constant memory --- pass the data through @ref MeshTools::owned() to
+get a mutable copy, if needed.
 
 @image html primitives-squarewireframe.png width=256px