fast-import doc: cat-blob and ls responses need to be consumed quickly

If fast-import's command pipe and the frontend's cat-blob/ls response
pipe are both filled, there can be a deadlock.  Luckily all existing
frontends consume any pending cat-blob/ls responses completely before
writing the next command.

Document the requirements so future frontend authors and users can be
spared from the problem, too.  It is not always easy to catch that
kind of bug by testing.

To set the scene, add some words of explanation to help the novice
understand that "cat-blob" and "ls" output are meant for consumption
by the frontend.

Reported-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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