am: document that Date: can appear as an in-body header

Similar to "From:" and "Subject:" already mentioned in the
documentation, "Date:" can also appear as an in-body header
to override the value in the e-mail headers.  Document it.

Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-am.txt

@@ -190,8 +190,8 @@ the commit, after stripping common prefix "[PATCH <anything>]".
 The "Subject: " line is supposed to concisely describe what the
 commit is about in one line of text.
 
-"From: " and "Subject: " lines starting the body override the respective
-commit author name and title values taken from the headers.
+"From: ", "Date: ", and "Subject: " lines starting the body override the
+respective commit author name and title values taken from the headers.
 
 The commit message is formed by the title taken from the
 "Subject: ", a blank line and the body of the message up to