git-shortlog(1): document behaviour of zero-width wrap

Commit 00d3947 (Teach --wrap to only indent without wrapping) added
special behaviour for a width of zero in the '-w' argument to
'git-shortlog' but this was not documented.  Fix this.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-shortlog.txt

@@ -44,6 +44,9 @@ OPTIONS
 	line of each entry is indented by `indent1` spaces, and the second
 	and subsequent lines are indented by `indent2` spaces. `width`,
 	`indent1`, and `indent2` default to 76, 6 and 9 respectively.
++
+If width is `0` (zero) then indent the lines of the output without wrapping
+them.
 
 
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