Mention @fn_gl and @def_gl in Coding Style manual.

doc/coding-style.dox

@@ -49,6 +49,21 @@ the operator is implemented (not of class in which the operator is
 implemented), thus efficiently connecting the two classes together in the
 documentation.
 
+@subsubsection documentation-commands-ref_gl Links to related OpenGL functions and definitions
+
+If an function touches OpenGL, related OpenGL functions should be documented
+in @c \@see block with @c \@fn_gl command. If only specific definition is used
+in the function, document it with @c \@def_gl command. Example usage:
+@code
+// @see @fn_gl{Enable}/@fn_gl{Disable} with @def_gl{TEXTURE_CUBE_MAP_SEAMLESS}
+inline static void setSeamless(bool enabled) {
+    enabled ? glEnable(GL_TEXTURE_CUBE_MAP_SEAMLESS) : glDisable(GL_TEXTURE_CUBE_MAP_SEAMLESS);
+}
+@endcode
+
+It produces link to the online manual, in this case @fn_gl{Enable}/@fn_gl{Disable}
+with @def_gl{TEXTURE_CUBE_MAP_SEAMLESS}.
+
 @subsubsection documentation-commands-extension Links to OpenGL extensions
 
 If an OpenGL extension is referenced in the documentation, it should be done