git-am.txt: reword extra headers in message body

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

Documentation/git-am.txt

@@ -121,10 +121,10 @@ 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.
 
-The body of the message (the rest of the message after the blank line
-that terminates the RFC2822 headers) can begin with "Subject: " and
-"From: " lines that are different from those of the mail header,
-to override the values of these fields.
+"From: " and "Subject: " lines starting the body (the rest of the
+message after the blank line terminating the RFC2822 headers)
+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