Improve documentation of face numbers

* doc/lispref/display.texi (Face Functions): Mention where the
face number is used and that it depends on the 'face' property
of the face symbol.  Improve indexing.
1 job for emacs-26 in 60 minutes and 1 second (queued for 4 seconds)
Status Job ID Name Coverage
failed #677


Name Stage Failure
test Test The script exceeded the maximum execution time set for the job
lisp/ses-tests.el:148:19:Warning: reference to free variable `A3'
lisp/ses-tests.el:156:9:Warning: assignment to free variable
lisp/ses-tests.el:172:16:Warning: reference to free variable `bar'
GEN lisp/ses-tests.log
ELC lisp/shadowfile-tests.elc
GEN lisp/shadowfile-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-f100a208 ...
ERROR: Job failed: execution took longer than 1h0m0s seconds