Clarify documentation for the new pgg method.

Clarify documentation for the new pgg method.
(encrypt-file-alist): Add PGG option.
(encrypt-insert-file-contents, encrypt-write-file-contents): Use PGG
functionality.  Abstract password key and messaging to external functions.
(encrypt-password-key, encrypt-get-passphrase-if-needed)
(encrypt-message-method-and-cipher): Add new convenience external functions.
(encrypt-pgg-encode-buffer, encrypt-pgg-decode-buffer)
(encrypt-pgg-process-buffer): Add PGG functionality glue.