Docs: reference G & U from Global & UserDef doc-strings

Include some context for how these structs are used as it's not so
clear where values are set/initialized.

source/blender/blenkernel/BKE_global.hh

@@ -16,6 +16,13 @@
 
 struct Main;
 
+/**
+ * Global data, typically accessed from #G.
+ * See: #BKE_blender_globals_init & #BKE_blender_globals_clear.
+ *
+ * \note This is run-time only but some global data is written
+ * to #FileGlobal which is used to initialize members of #Global.
+ */
 struct Global {
 
   /**

source/blender/makesdna/DNA_userdef_types.h

@@ -740,6 +740,12 @@ typedef struct bUserScriptDirectory {
   char dir_path[768]; /* FILE_MAXDIR */
 } bUserScriptDirectory;
 
+/**
+ * Main user preferences data, typically accessed from #U.
+ * See: #BKE_blendfile_userdef_from_defaults & #BKE_blendfile_userdef_read.
+ *
+ * \note This is either loaded from the file #BLENDER_USERPREF_FILE or from memory, see #U_default.
+ */
 typedef struct UserDef {
   DNA_DEFINE_CXX_METHODS(UserDef)