UI: clarify auto-save preference

- Grey out the option when factory settings have been loaded.
- Note it it's tool-top that it's ignored when factory settings
  have been loaded.

Suggestions by @JulienKaspar

release/scripts/startup/bl_ui/space_userpref.py

@@ -86,7 +86,9 @@ class USERPREF_MT_save_load(Menu):
 
         prefs = context.preferences
 
-        layout.prop(prefs, "use_preferences_save", text="Auto-Save Preferences")
+        row = layout.row()
+        row.active = not bpy.app.use_userpref_skip_save_on_exit
+        row.prop(prefs, "use_preferences_save", text="Auto-Save Preferences")
 
         layout.separator()
 

source/blender/makesrna/intern/rna_userdef.c

@@ -5820,7 +5820,10 @@ void RNA_def_userdef(BlenderRNA *brna)
   /* Preferences Flags */
   prop = RNA_def_property(srna, "use_preferences_save", PROP_BOOLEAN, PROP_NONE);
   RNA_def_property_boolean_sdna(prop, NULL, "pref_flag", USER_PREF_FLAG_SAVE);
-  RNA_def_property_ui_text(prop, "Save on Exit", "Save preferences on exit when modified");
+  RNA_def_property_ui_text(prop,
+                           "Save on Exit",
+                           "Save preferences on exit when modified "
+                           "(unless factory settings have been loaded)");
 
   prop = RNA_def_property(srna, "is_dirty", PROP_BOOLEAN, PROP_NONE);
   RNA_def_property_boolean_sdna(prop, NULL, "runtime.is_dirty", 0);