Update section heading conventions for libraries

Update section heading conventions for libraries

* doc/lispref/tips.texi (Comment Tips): Update information on section
  headings to reflect common usage.

Previously the tips stated that if the code is split up into multiple
sections, then that should be done by splitting up the ";;; Code:"
section into multiple sub-sections.

However about half the libraries in Emacs instead use multiple
top-level sections.  We update the tips (aka conventions) to allow
this common usage, but because it is awkward if there is a section
named "Code", which contains only some of the code instead of all of
it, we now recommend that that section should be empty in this case.

We cannot just give up on the "Code:" section/heading because that is
an old convention that is followed be nearly every library and because
it is likely that there are some utilities out there that depend on
its presence.

This was discussed in
https://lists.gnu.org/archive/html/emacs-devel/2020-07/msg00444.html
https://lists.gnu.org/archive/html/emacs-devel/2020-08/msg00001.html