doc: More explicit doc for EXTERN and LOCAL modes.
* doc/gpgme.texi (GPGME_KEYLIST_MODE_EXTERN), (GPGME_KEYLIST_MODE_LOCATE, GPGME_KEYLIST_MODE_LOCAL): Mention command line flags for CMS and OpenPGP Protocol.
This might help new users coming to gnupg to understand what
the flags do.