doc: add shortcut to "am --whitespace=<action>"

We refer readers of "git am --help" to "git apply --help" for many
options that are passed through, and most of them are simple
booleans, but --whitespace takes from a set of actions whose names
may slip users' minds.  Give a list of them in "git am --help" to
reduce one level of redirection only to find out what they are.

In the helper function to parse the available options, there was a
helpful comment reminding the developer to update list of <action>s
in the completion script. Mention the two documentation pages there
as well.

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

Documentation/git-am.txt

@@ -128,6 +128,9 @@ include::rerere-options.txt[]
 	These flags are passed to the 'git apply' (see linkgit:git-apply[1])
 	program that applies
 	the patch.
++
+Valid <action> for the `--whitespace` option are:
+`nowarn`, `warn`, `fix`, `error`, and `error-all`.
 
 --patch-format::
 	By default the command will try to detect the patch format

apply.c

@@ -78,7 +78,8 @@ static int parse_whitespace_option(struct apply_state *state, const char *option
 		return 0;
 	}
 	/*
-	 * Please update $__git_whitespacelist in git-completion.bash
+	 * Please update $__git_whitespacelist in git-completion.bash,
+	 * Documentation/git-apply.txt, and Documentation/git-am.txt
 	 * when you add new options.
 	 */
 	return error(_("unrecognized whitespace option '%s'"), option);