git-prompt.sh: update PROMPT_COMMAND documentation

The description of __git_ps1 function operating in two-arg mode was
not very clear.  It said "set PROMPT_COMMAND=__git_ps1" which is not
the right usage for this mode, followed by "To customize the prompt,
do this", giving a false impression that those who do not want to
customize it can get away with no-arg form, which was incorrect.

Make it clear that this mode always takes two arguments, pre and
post, with an example.

The straight-forward one should be listed as the primary usage, and
the confusing one should be an alternate for advanced users.  Swap
the order of these two.

Acked-by: Simon Oosthoek <s.oosthoek@xs4all.nl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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