Shaders: mention texture array use in overview docs.

It was only briefly mentioned in the multidraw section, which is far
from enough.

src/Magnum/Shaders/FlatGL.h

@@ -109,6 +109,10 @@ Common rendering setup:
 
 @snippet Shaders-gl.cpp FlatGL-usage-textured2
 
+If @ref Flag::TextureArrays is enabled, pass a @ref GL::Texture2DArray instance
+instead of @ref GL::Texture2D. By default layer @cpp 0 @ce is used, call
+@ref setTextureLayer() to pick a different texture array layer.
+
 For coloring the texture based on intensity you can use the @ref VectorGL
 shader. The 3D version of this shader is equivalent to @ref PhongGL with zero
 lights, however this implementation is much simpler and thus likely also
@@ -224,6 +228,10 @@ the @ref Shaders-FlatGL-colored "colored case at the top" would look like this:
 
 @snippet Shaders-gl.cpp FlatGL-ubo
 
+When uniform buffers with @ref Flag::TextureArrays are used,
+@ref Flag::TextureTransformation has to be enabled as well in order to supply
+the texture layer using @ref TextureTransformationUniform::layer.
+
 For a multidraw workflow enable @ref Flag::MultiDraw (and possibly
 @ref Flag::TextureArrays), supply desired material and draw count via
 @ref Configuration::setMaterialCount() and

src/Magnum/Shaders/MeshVisualizerGL.h

@@ -1531,7 +1531,7 @@ Consistently with the other shaders, textured object ID is also supported if
 the @ref TextureCoordinates attribute and bind an integer texture via
 @ref bindObjectIdTexture(). @ref Flag::TextureTransformation then enables
 texture transformation and @ref Flag::TextureArrays texture arrays for the
-object ID texture.
+object ID texture, with the layer selected using @ref setTextureLayer().
 
 If you enable @ref Flag::VertexId, the shader will use the color map to
 visualize how are vertices shared among primitives. That's useful for

src/Magnum/Shaders/PhongGL.h

@@ -82,6 +82,10 @@ Common rendering setup:
 
 @snippet Shaders-gl.cpp PhongGL-usage-texture2
 
+If @ref Flag::TextureArrays is enabled, pass @ref GL::Texture2DArray instances
+instead of @ref GL::Texture2D. By default layer @cpp 0 @ce is used, call
+@ref setTextureLayer() to pick a different texture array layer.
+
 @section Shaders-PhongGL-lights Light specification
 
 By default, the shader provides a single directional "fill" light, coming from
@@ -310,6 +314,10 @@ would look like this:
 
 @snippet Shaders-gl.cpp PhongGL-ubo
 
+When uniform buffers with @ref Flag::TextureArrays are used,
+@ref Flag::TextureTransformation has to be enabled as well in order to supply
+the texture layer using @ref TextureTransformationUniform::layer.
+
 For a multidraw workflow enable @ref Flag::MultiDraw (and possibly
 @ref Flag::TextureArrays) and supply desired light, material and draw count via
 @ref Configuration::setLightCount(),