Improve the documentation of setting up fontsets

* doc/lispref/display.texi (Fontsets): Improve the accuracy of a
cross-reference to "Character Properties".

* doc/emacs/mule.texi (Fontsets, Modifying Fontsets): Improve the
documentation of fontsets and how to modify them.
1 job for emacs-27 in 90 minutes and 1 second (queued for 3 seconds)
Status Job ID Name Coverage
failed #6073


Name Stage Failure
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/eshell/eshell-tests.log
ELC lisp/faces-tests.elc
GEN lisp/faces-tests.log
ELC lisp/ffap-tests.elc
GEN lisp/ffap-tests.log
ELC lisp/filenotify-tests.elc
GEN lisp/filenotify-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-d0b76032 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds