Advertise obsolete library pgg.el less

Advertise obsolete library pgg.el less

* doc/misc/gnus-faq.texi (FAQ 8-2):
* doc/misc/gnus.texi (Security):
* doc/misc/message.texi (Using OpenPGP, PGP Compatibility):
* doc/misc/mh-e.texi (Reading PGP, Sending PGP):
* lisp/gnus/mml2015.el (mml2015-use):
* lisp/info-look.el (mapc):
* lisp/mh-e/mh-e.el (mh-mml-method-default): Advertise obsolete
library pgg.el less.

* lisp/mh-e/mh-identity.el (mh-identity-pgg-default-user-id): Rename
from 'mh-identity-gpg-default-user-id', and make the old name into
obsolete variable alias.  Update all uses.