doc: clarify how -S works

The existing text was very vague about what exactly it means
for difference to "contain" a change. This seems to cause
confusion on the mailing list every month or two.

To fix it we:

  1. use "introduce or remove an instance of" instead of
     "contain"

  2. point the user to gitdiffcore(7), which contains a more
     complete explanation

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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