Correct doc about the Git module verify_commit param

All that is required to verify the signature is that the matching
public key is present in the remote user's keyring. There is no need
for GnuPG to explicitly trust the authenticity of the key.

Not Ansible specific, but rather the behavior of the `git verify-commit`
and the `git verify-tag` command line invocations.

lib/ansible/modules/source_control/git.py

@@ -183,7 +183,7 @@ options:
             - if C(yes), when cloning or checking out a C(version) verify the
               signature of a GPG signed commit. This requires C(git) version>=2.1.0
               to be installed. The commit MUST be signed and the public key MUST
-              be trusted in the GPG trustdb.
+              be present in the GPG keyring.
 
     archive:
         required: false