Cleanup: doxy comments

Use doxy references to function and enums,
also correct some names which became out of sync.

source/blender/gpu/GPU_immediate.h

@@ -124,8 +124,10 @@ void immUniformColor3ubv(const unsigned char rgb[3]);
 void immUniformColor3ubvAlpha(const unsigned char rgb[3], unsigned char a);
 void immUniformColor4ubv(const unsigned char rgba[4]);
 
-/* Extend immBindProgram to use Blender’s library of built-in shader programs.
- * Use immUnbindProgram() when done. */
+/**
+ * Extend #immBindProgram to use Blender’s library of built-in shader programs.
+ * Use #immUnbindProgram() when done.
+ */
 void immBindBuiltinProgram(eGPUBuiltinShader shader_id);
 
 /* Extend immUniformColor to take Blender's themes */