Documentation: explain "mboxrd" pretty format

The "mboxrd" pretty format was introduced in 9f23e04 (pretty: support
"mboxrd" output format, 2016-06-05) but wasn't mentioned in the
documentation.

Signed-off-by: Emma Brooks <me@pluvano.com>
Acked-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -83,6 +83,12 @@ placeholders, its output is not affected by other options like
 
 	  <full commit message>
 
+* 'mboxrd'
++
+Like 'email', but lines in the commit message starting with "From "
+(preceded by zero or more ">") are quoted with ">" so they aren't
+confused as starting a new commit.
+
 * 'raw'
 +
 The 'raw' format shows the entire commit exactly as