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>