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>