Appropriately reference MeshTools::primitiveCount().

Not from GL::Mesh, because it doesn't accept GL-specific enums.

src/Magnum/Mesh.h

@@ -57,6 +57,8 @@ presence.
 For D3D, corresponds to @m_class{m-doc-external} [D3D_PRIMITIVE_TOPOLOGY](https://docs.microsoft.com/en-us/windows/win32/api/d3dcommon/ne-d3dcommon-d3d_primitive_topology);
 for Metal, corresponds to @m_class{m-doc-external} [MTLPrimitiveType](https://developer.apple.com/documentation/metal/mtlprimitivetype?language=objc).
 See documentation of each value for more information about the mapping.
+
+@see @ref MeshTools::primitiveCount(MeshPrimitive, UnsignedInt)
 */
 enum class MeshPrimitive: UnsignedInt {
     /* Zero reserved for an invalid type (but not being a named value) */

src/Magnum/Trade/MeshData.h

@@ -1051,7 +1051,8 @@ class MAGNUM_TRADE_EXPORT MeshData {
          * @ref vertexCount() which returns count of elements in every
          * @ref attribute() array, and @ref attributeCount() which returns
          * count of different per-vertex attribute arrays.
-         * @see @ref isIndexed(), @ref indexType()
+         * @see @ref isIndexed(), @ref indexType(),
+         *      @ref MeshTools::primitiveCount(MeshPrimitive, UnsignedInt)
          */
         UnsignedInt indexCount() const;
 
@@ -1127,6 +1128,7 @@ class MAGNUM_TRADE_EXPORT MeshData {
          * count of elements in the @ref indices() array, and
          * @ref attributeCount() which returns count of different per-vertex
          * attribute arrays.
+         * @see @ref MeshTools::primitiveCount(MeshPrimitive, UnsignedInt)
          */
         UnsignedInt vertexCount() const { return _vertexCount; }