Some doc edits related to desktop-restore-frames

Some doc edits related to desktop-restore-frames

* doc/emacs/misc.texi (Saving Emacs Sessions):
Be briefer about desktop's handling of frames.
The manual does not need to mention every option, nor should
it just repeat the doc-strings.

* lisp/desktop.el (desktop-restore-frames)
(desktop-restore-in-current-display, desktop-restore-forces-onscreen)
(desktop-restore-reuses-frames): Doc tweaks.

* etc/NEWS: Related edits.