Documentation: Describe "git diff <blob> <blob>" separately

As it was not a common operation, it was described as if it is a
side note for the more common two-commit variant, but this mode
behaves very differently, e.g. it does not make any sense to ask
recursive behaviour, or give the command a pathspec.

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

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