rev-list: clarify git-log default date format

The documentation mistakenly said that the default format was
similar to RFC 2822 format and tried to specify it by enumerating
differences, which had two problems:

 * There are some more differences from the 2822 format that are not
   mentioned; worse yet

 * The default format is not modeled after RFC 2822 format at all.
   As can be seen in f80cd78 (date.c: add "show_date()" function.,
   2005-05-06), it is a derivative of ctime(3) format.

Stop saying that it is similar to RFC 2822, and rewrite the
description to explain the format without requiring the reader to
know any other format.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/rev-list-options.txt

@@ -1100,12 +1100,12 @@ preferred format.  See the `strftime` manual for a complete list of
 format placeholders. When using `-local`, the correct syntax is
 `--date=format-local:...`.
 
-`--date=default` is the default format, and is similar to
-`--date=rfc2822`, with a few exceptions:
+`--date=default` is the default format, and is based on ctime(3)
+output.  It shows a single line with three-letter day of the week,
+three-letter month, day-of-month, hour-minute-seconds in "HH:MM:SS"
+format, followed by 4-digit year, plus timezone information, unless
+the local time zone is used, e.g. `Thu Jan 1 00:00:00 1970 +0000`.
 --
-	- there is no comma after the day-of-week
-
-	- the time zone is omitted when the local time zone is used
 
 ifdef::git-rev-list[]
 --header::