manual/message.texi: Fix english and clarify.

Rewrite the first paragraph to talk about users not humans,
and to use correct English.

Clarify that it is the mapping of messages to IDs that
impacts the design of the message translation API.

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2013-05-07  Carlos O'Donell  <carlos@redhat.com>

	* manual/message.texi (Message Translation): Talk about users.
	Message to key mapping impacts design.