doc: clarify support for queries of GL program resources.

doc/opengl-mapping.dox

@@ -213,11 +213,7 @@ OpenGL function                         | Matching API
 @fn_gl{GetProgramInterface}             | |
 @fn_gl{GetProgramPipeline}              | |
 @fn_gl{GetProgramPipelineInfoLog}       | |
-@fn_gl{GetProgramResource}              | |
-@fn_gl{GetProgramResourceIndex}         | |
-@fn_gl{GetProgramResourceLocation}      | |
-@fn_gl{GetProgramResourceLocationIndex} | |
-@fn_gl{GetProgramResourceName}          | |
+@fn_gl{GetProgramResource}, \n @fn_gl{GetProgramResourceIndex}, \n @fn_gl{GetProgramResourceLocation}, \n @fn_gl{GetProgramResourceLocationIndex}, \n @fn_gl{GetProgramResourceName} | not queryable, set them in shader code directly as described in @ref GL::AbstractShaderProgram docs
 @fn_gl{GetProgramStage}                 | |
 @fn_gl{GetQueryIndexed}                 | |
 @fn_gl{GetQueryObject}                  | @ref GL::AbstractQuery::result()

doc/opengl-support.dox

@@ -540,12 +540,17 @@ add any performance gains, is not supported in Magnum. See also
 -   Shader compiler is assumed to be present (`GL_SHADER_COMPILER` returning
     true), as all desktop GL implementations and also ES3 are required to
     support it.
--   Querying shader attribute locations (as opposed to setting them) is not
-    supported, as newer GL versions encourage setting them explicitly in shader
-    code.
+-   Querying shader attribute locations using @fn_gl_keyword{GetAttribLocation}
+    and other resources using @fn_gl_keyword{GetProgramResource},
+    @fn_gl_keyword{GetProgramResourceIndex},
+    @fn_gl_keyword{GetProgramResourceLocation},
+    @fn_gl_keyword{GetProgramResourceLocationIndex}
+    or @fn_gl_keyword{GetProgramResourceName} (as opposed to setting them) is
+    not supported, as newer GL versions encourage setting them explicitly in
+    shader code.
 -   Direct vertex data specification (as opposed to using buffers) is not
     supported, as it encourages bad practices.
--   @fn_gl{glPixelStore} with @def_gl{PACK_SWAP_BYTES} / @def_gl{UNPACK_SWAP_BYTES}.
+-   @fn_gl{PixelStore} with @def_gl{PACK_SWAP_BYTES} / @def_gl{UNPACK_SWAP_BYTES}.
     Too much burden imposed on every API that deals with pixel storage. Use
     @ref Corrade::Utility::Endianness instead. Note that this state is also not
     tracked, so it won't get reset in case a 3rd party code enables it.