Check and document the options to prevent mistakes.

Check and document the options to prevent mistakes.

When multiple recipients are given to git-send-email on the same
--cc line the code does not properly handle it.

Full and proper parsing of the email addresses so I can detect
which commas mean a new email address is more than I care to implement.

In particular this email address: "bibo,mao" <bibo.mao@intel.com>
must not be treated as two email addresses.

So this patch simply treats all commas in recipient lists as
an error and fails if one is given.

At the same time it documents that git-send-email wants multiple
instances of --cc specified on the command line if you want to
cc multiple recipients.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>