doc: clarify how exit status of post-checkout hook is used

Because the hook runs after the main checkout operation finishes, it
cannot affect what branch will be the current branch, what paths are
updated in the working tree, etc., which was described as "cannot
affect the outcome of 'checkout'".

However, the exit status of the hook is used as the exit status of
the 'checkout' command and is observable by anybody who spawned the
'checkout', which was missing from the documentation.  Fix this.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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