GL: cross-reference renderbuffer and texture max sample count queries.

Because, searching for maxSamples() and finding it only for a
Renderbuffer, I got an impression that there isn't an equivalent for
textures. There is, so link to that in the docs.

src/Magnum/GL/AbstractTexture.h

@@ -175,6 +175,7 @@ class MAGNUM_GL_EXPORT AbstractTexture: public AbstractObject {
          * @see @fn_gl{Get} with @def_gl_keyword{MAX_COLOR_TEXTURE_SAMPLES}
          * @requires_gles30 Not defined in OpenGL ES 2.0.
          * @requires_gles Multisample textures are not available in WebGL.
+         * @see @ref MultisampleTexture, @ref Renderbuffer::maxSamples()
          */
         static Int maxColorSamples();
 
@@ -188,6 +189,7 @@ class MAGNUM_GL_EXPORT AbstractTexture: public AbstractObject {
          * @see @fn_gl{Get} with @def_gl_keyword{MAX_DEPTH_TEXTURE_SAMPLES}
          * @requires_gles30 Not defined in OpenGL ES 2.0.
          * @requires_gles Multisample textures are not available in WebGL.
+         * @see @ref MultisampleTexture, @ref Renderbuffer::maxSamples()
          */
         static Int maxDepthSamples();
 
@@ -201,6 +203,7 @@ class MAGNUM_GL_EXPORT AbstractTexture: public AbstractObject {
          * @see @fn_gl{Get} with @def_gl_keyword{MAX_INTEGER_SAMPLES}
          * @requires_gles30 Not defined in OpenGL ES 2.0.
          * @requires_gles Multisample textures are not available in WebGL.
+         * @see @ref MultisampleTexture, @ref Renderbuffer::maxSamples()
          */
         static Int maxIntegerSamples();
         #endif

src/Magnum/GL/Renderbuffer.h

@@ -93,6 +93,9 @@ class MAGNUM_GL_EXPORT Renderbuffer: public AbstractObject {
          *      @def_gl_keyword{MAX_SAMPLES}
          * @requires_webgl20 Multisample framebuffers are not available in
          *      WebGL 1.0.
+         * @see @ref AbstractTexture::maxColorSamples(),
+         *      @ref AbstractTexture::maxDepthSamples(),
+         *      @ref AbstractTexture::maxIntegerSamples()
          */
         static Int maxSamples();
         #endif