Documentation/stash: remove mention of git reset --hard

Documentation/stash: remove mention of git reset --hard

Don't mention git reset --hard in the documentation for git stash save.
It's an implementation detail that doesn't matter to the end user and
thus shouldn't be exposed to them.  In addition it's not quite true for
git stash -p, and will not be true when a filename argument to limit the
stash to a few files is introduced.

Signed-off-by: Thomas Gummerer <redacted>
Signed-off-by: Junio C Hamano <redacted>