pathspec doc: parse_pathspec does not maintain references to args

pathspec doc: parse_pathspec does not maintain references to args

The command line arguments passed to main() are valid for the life of
a program, but the same is not true for all other argv-style arrays
(e.g.  when a caller creates an argv_array).  Clarify that
parse_pathspec does not rely on the argv passed to it to remain valid.

This makes it easier to tell that callers like "git rev-list --stdin"
are safe and ensures that that is more likely to remain true as the
implementation of parse_pathspec evolves.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>