One more improvement of left/right-fringe display spec docs

* doc/lispref/display.texi (Fringe Bitmaps): Yet another
clarification of how to use FACE in left/right-fringe display
1 job for emacs-27 in 14 seconds (queued for 2 seconds)
Status Job ID Name Coverage
failed #6566


Name Stage Failure
test-all Test
error: cannot lock ref 'refs/remotes/origin/master': Unable to create '/builds/emacs/emacs/.git/refs/remotes/origin/master.lock': File exists.

Another git process seems to be running in this repository, e.g.
an editor opened by 'git commit'. Please make sure all processes
are terminated then try again. If it still fails, a git process
may have crashed in this repository earlier:
remove the file manually to continue.
fatal: failed to run reflog
ERROR: Job failed: exit code 1