More proofreading of the Emacs manual

More proofreading of the Emacs manual

* doc/emacs/macos.texi (Mac / GNUstep Basics): Fix typos.  Improve
indexing.
(Mac / GNUstep Basics, Mac / GNUstep Customization)
(Mac / GNUstep Events): Improve indexing.

* doc/emacs/rmail.texi (Rmail): Mention Gnus.
(Rmail Basics, Rmail Scrolling): Improve cross-references and
indexing.
(Rmail Motion): Minor improvements.
(Rmail Inbox): Remove outdated text about Babyl format.
(Rmail Files): Formatting changes.  Comment out
set-rmail-inbox-list in the table of commands, as it is commented
out in the text that follows.
(Rmail Output, Rmail Labels, Rmail Reply, Rmail Make Summary):
Minor copyedits.
(Rmail Summary Edit): Document
rmail-summary-scroll-between-messages.
(Rmail Editing): Improve indexing.
(Movemail): Fix typos.
(Remote Mailboxes): Use "POP3" rather than "POP".