Merge branch 'kf/gpg-sig-verification-doc' into maint

Documentation for "git merge --verify-signatures" has been updated
to clarify that the signature of only the commit at the tip is
verified.  Also the phrasing used for signature and key validity is
adjusted to align with that used by OpenPGP.

* kf/gpg-sig-verification-doc:
Documentation: clarify signature verification