Improve documentation of features that use the fringes

* doc/emacs/display.texi (Fringes): Add cross-reference to
where indicate-empty-lines is described.
(Useless Whitespace): Add an @anchor for a more accurate
cross-reference in "Fringes".
1 job for emacs-26 in 39 minutes and 28 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #2947
test

00:39:28

 
Name Stage Failure
failed
test Test
-----------------------
Files examined: 191
Ran 2621 tests, 2561 results as expected, 1 unexpected, 59 skipped
1 files contained unexpected results:
src/process-tests.log
make[2]: *** [check-doit] Error 1
make[1]: *** [check] Error 2
make: *** [check] Error 2
ERROR: Job failed: exit code 1