Documentation: clarify effects of -- <path> arguments

Documentation: clarify effects of -- <path> arguments

'git log -- <path>' does not "show commits that affect the specified
paths" in a literal sense unless --full-history is given (for example,
a file that only existed on a side branch will turn up no commits at
all!).

Reword it to specify the actual intent of the filtering, and point to
the "History Simplification" section.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>