gpgv: Improve documentation for keyring choices

gpgv: Improve documentation for keyring choices

* doc/gpgv.texi: Improve documentation for keyring choices

--

From the existing documentation, it's not clear whether the default
keyring will always be mixed into the set of keyrings, or whether it
will be skipped if a --keyring is present.  The updated text here
attempts to describe the keyring selection logic more completely.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
(cherry picked from commit a7b2a87f940dba078867c44f1f50d46211d51719)

Gbp-Pq: Topic from-2.2.14
Gbp-Pq: Name gpgv-Improve-documentation-for-keyring-choices.patch