Suggested by Deck Pickard <deck.r.pickard@gmail.com>.

* guix/scripts/build.scm (show-build-options-help): Document
  --max-jobs.
  (set-build-options-from-command-line): Pass #:max-build-jobs.
  (%standard-build-options): Add --max-jobs.
* doc/guix.texi (Invoking guix-daemon): Document the meaning of
  '--max-jobs 0'.
  (Invoking guix build): Document --max-jobs, with a reference to
  "Invoking guix-daemon'.