More updates to Text chapter of Emacs manual.

More updates to Text chapter of Emacs manual.

* text.texi (Pages): Mention how formfeed chars are displayed.
(Auto Fill): Note convention for calling auto-fill-mode from Lisp.
Describe adaptive filling more precisely.
(Fill Commands): Note that filling removes excess whitespace.
(Text Mode): Note auto-mode-alist entries for Text mode.  TAB is
now bound to indent-for-tab-command in Text mode.
(Outline Mode): Copyedits.
(Outline Visibility): Note that Reveal mode is a buffer-local
minor mode.

* modes.texi (Major Modes): Move note about checking major-mode in
a hook function here, from Text mode.