glib: document restrictions on various foreach() functions

Some foreach() functions allow you to modify the object they are
iterating, and others don't, but the docs were not generally clear
about this.

https://bugzilla.gnome.org/show_bug.cgi?id=724383

glib/garray.c

@@ -1503,7 +1503,8 @@ g_ptr_array_sort_with_data (GPtrArray        *array,
  * @func: the function to call for each array element
  * @user_data: user data to pass to the function
  * 
- * Calls a function for each element of a #GPtrArray.
+ * Calls a function for each element of a #GPtrArray. @func must not
+ * add elements to or remove elements from the array.
  *
  * Since: 2.4
  */