Another set of changes for the manual

Another set of changes for the manual

* doc/emacs/programs.texi (Program Indent): Add a cross-reference
to elisp's description of 'pp'.
(Program Modes): Add a few more programming modes.  Add index
entries.
(Basic Indent, Multi-line Indent, C Indent, Comment Commands)
(Manipulating Comments): Fix markup of keyboard commands.
* doc/emacs/search.texi (Regexps): Add an example with non-ASCII
characters.  Suggested by Michael Albinus <michael.albinus@gmx.de>
in emacs-manual-bugs@gnu.org.

* doc/lispref/display.texi (Display Tables): Fix the description
of the 5th extra slot of the display table.  (Bug#13473)

* doc/emacs/regs.texi (Registers): Simplify wording.

* doc/emacs/custom.texi (Init Non-ASCII): Remove outdated text
about perils of encoded keyboard input.