doc: Proposed note about symlinks in section "invoking guix pull"

[swedebugia]

Hi

The manual reads: 

"This ~/.config/guix/current profile works like any other profile created by guix package (see Invoking guix package). That is, you can list generations, roll back to the previous generation—i.e., the previous Guix—and so on:

$ guix package -p ~/.config/guix/current --roll-back
switched from generation 3 to 2
$ guix package -p ~/.config/guix/current --delete-generations=1
deleting /home/charlie/.config/guix/current-1-link"

Users without a working guix is not helped by this.
Ie they get errors when trying to run guix package -p ...:
"Missing guile-gcrypt bla bla"

I suggest a note below this saying:
"Note: if your have wound up with a faulty guix after a "guix pull" you can manually run an older version by looking at the symlinks in your /user/home/.config/guix/ directory.
E.g. if it lists:
$ ls -l /root/.config/guix/
total 0
lrwxrwxrwx 1 root root 33 Nov  4 01:24 current -> /root/.config/guix/current-1-link
...
Then your faulty guix is in current-1-link, and your former probably well working guix is in an older symlink, e.g. current-2-link.
To run the guix-version before the last pull in the example above, run:
/root/.config/guix/current-2-link/bin/guix

Now go ahead and use that older guix to roll back as described above."

What do you think?

--
Cheers Swedebugia