doc: git-reset: clarify `git reset <pathspec>`

doc: git-reset: clarify `git reset <pathspec>`

From user feedback:

- Continued confusion about the terms "tree-ish" and "pathspec"
- The word "hunks" is confusing folks, use "changes" instead.
- On the part about `git restore`, there were a few comments to the
  effect of "wait, this doesn't actually update any files? What? Why?"
  Be more direct that `git reset` does not update files: there's no
  obvious reason to suggest that folks use `git reset` followed by `git
  restore`, instead suggest just using `git restore`.

Continue avoiding the use of the word "reset" to
describe what "git reset" does.

Signed-off-by: Julia Evans <julia@jvns.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>