Improve "Buffers" in the user manual

Improve "Buffers" in the user manual

Suggested by John Cummings <john@rootabega.net> in
emacs-manual-bugs@gnu.org:
* doc/emacs/buffers.texi (Select Buffer): Add cross-references to
"Windows" and "Frames".
(Misc Buffer, Kill Buffer): Use BUFFER consistently in commands
that accept buffer names.
(Select Buffer): Improve description of "M-g M-g".  Improve
wording.
(Misc Buffer, Kill Buffer): Improve wording.
(Kill Buffer): Fix the response required by kill-some-buffers.
Mention customizable options that control what clean-buffer-list
does.
(Several Buffers): More detail about
'Buffer-menu-unmark-all-buffers'.