Finalize documentation of 'custom-prompt-customize-unsaved-options'

* doc/emacs/custom.texi (Saving Customizations): Index the new
function 'custom-prompt-customize-unsaved-options'.

* etc/NEWS: Mention when 'custom-prompt-customize-unsaved-options'
is useful.

doc/emacs/custom.texi

@@ -403,6 +403,8 @@ customizations in your initialization file.  This is because saving
 customizations from such a session would wipe out all the other
 customizations you might have on your initialization file.
 
+@cindex unsaved customizations, reminder to save
+@findex custom-prompt-customize-unsaved-options
   Please note that any customizations you have not chosen to save for
 future sessions will be lost when you terminate Emacs.  If you'd like
 to be prompted about unsaved customizations at termination time, add

etc/NEWS

@@ -135,8 +135,10 @@ frames.
 Works for functions, variables, faces, etc.  It is bound to `C-h o' by
 default.
 
++++
 ** New function `custom-prompt-customize-unsaved-options' checks for
-unsaved customizations and prompts user to customize (if found).
+unsaved customizations and prompts user to customize (if found).  It
+is intended for adding to 'kill-emacs-query-functions'.
 
 +++
 ** Network security (TLS/SSL certificate validity and the like) is