vlc_opengl: document vlc_gl_GetProcAddress

Document that the function must be called with a context enabled. Even
if most low-level implementation will allow getting an address without
a context, so as to keep the same symbol across contexts, some other
common implementations like glfw[^1] will require an active context.

[^1]: https://www.glfw.org/docs/3.3/group__context.html#ga35f1837e6f666781842483937612f163

include/vlc_opengl.h

@@ -134,6 +134,24 @@ static inline picture_t *vlc_gl_SwapOffscreen(vlc_gl_t *gl)
     return gl->ops->swap_offscreen(gl);
 }
 
+/**
+ * Fetch a symbol or pointer function from the OpenGL implementation.
+ *
+ * Return a pointer from the OpenGL implementation, which can be part of
+ * either the underlying OpenGL provider or an OpenGL function matching
+ * the version requested.
+ *
+ * If the symbol name is not matching the underlying implementation of
+ * OpenGL, an invalid pointer or NULL can be returned.
+ *
+ * @note This function must be called between MakeCurrent and ReleaseCurrent.
+ *
+ * @param gl the OpenGL provider to fetch the function from
+ * @param name the symbol name to fetch from the implementation
+ *
+ * @return A pointer corresponding to the symbol, or a potentially invalid
+ *         value or NULL in case of error.
+ */
 static inline void *vlc_gl_GetProcAddress(vlc_gl_t *gl, const char *name)
 {
     return gl->ops->get_proc_address(gl, name);