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>

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