doc: Use clearer notations for permissions.

doc: Use clearer notations for permissions.

* doc/lispref/files.texi (File Attributes, Changing Files):
Use a more-natural notation for octal numbers.
* doc/misc/gnus.texi (Mail Source Customization, Mail Back End Variables):
Use octal notation for file permissions, which are normally
thought of in octal.
(Mail Back End Variables): Use more-plausible modes in example.