gerror: Add a docs paragraph about not displaying errors verbatim in UI

It’s confusing and often doesn’t help the user. Match the error code and
come up with a more UI-appropriate error message.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

glib/gerror.c

@@ -114,6 +114,13 @@
  * function (the file being opened, or whatever - though in the
  * g_file_get_contents() case, the @message already contains a filename).
  *
+ * Note, however, that many error messages are too technical to display to the
+ * user in an application, so prefer to use g_error_matches() to categorize errors
+ * from called functions, and build an appropriate error message for the context
+ * within your application. Error messages from a #GError are more appropriate
+ * to be printed in system logs or on the command line. They are typically
+ * translated.
+ *
  * When implementing a function that can report errors, the basic
  * tool is g_set_error(). Typically, if a fatal error occurs you
  * want to g_set_error(), then return immediately. g_set_error()