Rename comint-fl-* to comint-fontify-input-* and mention it in NEWS

Rename comint-fl-* to comint-fontify-input-* and mention it in NEWS

* lisp/comint.el
(comint-fontify-input-mode):
(comint--fontify-input-saved-jit-lock-contextually):
(comint--fontify-input-on):
(comint--fontify-input-off):
(comint--fontify-input-ppss-flush-indirect):
(comint--fontify-input-fontify-region): Replace comint-fl-* with
comint-fontify-input-*.

* lisp/ielm.el (ielm-fontify-input-enable):
(ielm-fontify-input-enable):
(ielm-indirect-setup-hook):
(inferior-emacs-lisp-mode): Replace comint-fl-* with
comint-fontify-input-*.

* lisp/shell.el (shell-comint-fl-enable):
(shell-mode): Replace comint-fl-* with comint-fontify-input-*.

* etc/NEWS: Mention the new modes and how to disable or enable
them (bug#58169).