log --format: document %w

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -135,6 +135,10 @@ The placeholders are:
 - '%m': left, right or boundary mark
 - '%n': newline
 - '%x00': print a byte from a hex code
+- '%w([<w>[,<i1>[,<i2>]]])': switch line wrapping, like the -w option of
+  linkgit:git-shortlog[1].  NOTE: Color placeholders (`%C*`) are not
+  recognized as having no width, so they should not be put into wrapped
+  sections.
 
 NOTE: Some placeholders may depend on other options given to the
 revision traversal engine. For example, the `%g*` reflog options will