Documentation: Document signature showing options

The pretty formats for GPG signatures were introduced but never
documented. Use the documentation from the commit that introduced them.
Do the same for the --show-signature option added to git log and
friends.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -130,6 +130,9 @@ The placeholders are:
 - '%b': body
 - '%B': raw body (unwrapped subject and body)
 - '%N': commit notes
+- '%GG': raw verification message from GPG for a signed commit
+- '%G?': show either "G" for Good or "B" for Bad for a signed commit
+- '%GS': show the name of the signer for a signed commit
 - '%gD': reflog selector, e.g., `refs/stash@{1}`
 - '%gd': shortened reflog selector, e.g., `stash@{1}`
 - '%gn': reflog identity name

Documentation/pretty-options.txt

@@ -66,3 +66,7 @@ being displayed. Examples: "--notes=foo" will show only notes from
 --[no-]standard-notes::
 	These options are deprecated. Use the above --notes/--no-notes
 	options instead.
+
+--show-signature::
+	Check the validity of a signed commit object by passing the signature
+	to `gpg --verify` and show the output.