trailer --parse docs: add explanation for its usefulness

For users who are skimming the docs to go straight to the individual
breakdown of each flag, it may not be clear why --parse is a convenience
alias (without them also looking at the other options that --parse turns
on). To save them the trouble of looking at the other options (and
computing what that would mean), describe a summary of the overall
effect.

Similarly update the area when we first mention --parse near the top of
the doc.

Signed-off-by: Linus Arver <linusa@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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