Adjudicate review comments for the "Text" chapter of user manual

Adjudicate review comments for the "Text" chapter of user manual

* doc/emacs/text.texi (Text): Rearrange text for clarity.  Add
cross-reference to the Org manual.  Make the cross-reference to
Outline Mode appear in online manuals as well (the conditional was
a forgotten remnant from time immemoriam).
(Paragraphs): Add a note that 'paragraph-start' and
'paragraph-separate' should not be anchored.
(Auto Fill): Remove redundant text.  Suggested by Petteri
Hintsanen <petterih@iki.fi> in emacs-manual-bugs@gnu.org.