Documentation: describe the scissors mark support of "git am"

Describe what a scissors mark looks like, and explain in what situation
it is often used.

Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-am.txt

@@ -128,10 +128,18 @@ 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 (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.
+A line that mainly consists of scissors (either ">8" or "8<") and
+perforation (dash "-") marks is called a scissors line, and is used to
+request the reader to cut the message at that line.  If such a line
+appears in the body of the message before the patch, everything before it
+(including the scissors line itself) is ignored. This is useful if you
+want to begin your message in a discussion thread with comments and
+suggestions on the message you are responding to, and to conclude it with
+a patch submission, separating the discussion and the beginning of the
+proposed commit log message with a scissors line.
+
+"From: " 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