More updates of documentation for Emacs 30

More updates of documentation for Emacs 30

* etc/NEWS: Improve wording and move/mark entries.

* doc/lispref/positions.texi (List Motion): Document
'forward-sentence-function'.
* doc/lispref/functions.texi (What Is a Function): Document
'primitive-function-p' and 'cl-functionp'.
* doc/emacs/misc.texi (Saving Emacs Sessions): Improve wording and
indexing.
* doc/lispref/tips.texi (Documentation Tips): Adapt to the new
value of 'emacs-lisp-docstring-fill-column'.
* doc/emacs/fixit.texi (Spelling): Document
'flyspell-check-changes'.