Yest another round of manual copyedits

Yest another round of manual copyedits

* doc/emacs/fixit.texi (Transpose, Spelling): Minor stylistic
changes.  Suggested by myq larson <myqlarson@gmail.com> in
emacs-manual-bugs@gnu.org.

* doc/emacs/calendar.texi (Appointments, Time Intervals): Mention
relevant Org features.  Suggested by Alex Branham
<alex.branham@gmail.com> in emacs-manual-bugs@gnu.org.

* doc/emacs/dired.texi (Operating on Files)
(Shell Commands in Dired, Image-Dired): Minor stylistic edits.
Suggested by Francis Wright <f.j.wright@live.co.uk> in
emacs-manual-bugs@gnu.org.

* doc/emacs/commands.texi (User Input): Explain "C-M-a".
Suggested by Martin Luethi <martin.luethi@geo.uzh.ch> in
emacs-manual-bugs@gnu.org.