doc: make clear --assume-unchanged's user contract

doc: make clear --assume-unchanged's user contract

Many users misunderstand the --assume-unchanged contract, believing
it means Git won't look at the flagged file.

Be explicit that the --assume-unchanged contract is by the user that
they will NOT change the file so that Git does not need to look (and
expend, for example, lstat(2) cycles)

Mentioning "Git stops checking" does not help the reader, as it is
only one possible consequence of what that assumption allows Git to
do, but

   (1) there are things other than "stop checking" that Git can do
       based on that assumption; and
   (2) Git is not obliged to stop checking; it merely is allowed to.

Also, this is a single flag bit, correct the plural to singular, and
the verb, accordingly.

Drop the stale and incorrect information about "poor-man's ignore",
which is not what this flag bit is about at all.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>