fetch, pull doc: correct description of '--set-upstream'

fetch, pull doc: correct description of '--set-upstream'

The '--set-upstream' option to `git fetch` (which is also accepted by
`git pull` and passed through to the underlying `git fetch`) allows
setting the upstream configuration for the current branch. This was
added in 24bc1a1292 (pull, fetch: add --set-upstream option,
2019-08-19).

However, the documentation for that option describes its action as 'If
the remote is fetched successfully, pull and add upstream (tracking)
reference [...]', which is wrong because this option does not cause
neither `git fetch` nor `git pull` to pull: `git fetch` does not pull
and `git pull` always pulls.

Fix the description of that option.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>