doc: Better explain GC roots.

Fixes <https://bugs.gnu.org/24194>.
Reported by J. Eppler <j.eppler@openmailbox.org>.

* doc/guix.texi (Invoking guix-daemon): Add index entries for GC roots.
Add xref to "Invoking guix gc".
(Invoking guix gc): Add the string "GC roots"; mention /var/guix/gcroots.
(Additional Build Options): Add index entries.  Better explain --root.
(Invoking guix environment): Add xref to "Invoking guix gc".