Updates to Mark chapter in Emacs manual.

Updates to Mark chapter in Emacs manual.

* mark.texi (Mark): Clarify description of disabled Transient Mark
mode.
(Setting Mark): Document prefix arg for C-x C-x.  Document primary
selection changes.  Mention that commands like C-y set the mark.
(Marking Objects): Add xref to Words node.  Note that mark-word
and mark-sexp also have the "extend region" behavior.
(Using Region): Mention M-$ in the table.  Document
mark-even-if-inactive here instead of in Mark Ring.
(Mark Ring): Move mark-even-if-inactive to Using Region.  Take
note of the "Mark Set" behavior.
(Disabled Transient Mark): Rename from "Persistent Mark"
(Bug#9688).  Callers changed.

* programs.texi (Expressions):
* text.texi (Words): Defer to Marking Objects for mark-word doc.