Tweak C-x TAB behavior changes, and update docs.

Tweak C-x TAB behavior changes, and update docs.

* lisp/indent.el (indent-rigidly-map): Add docstring, and move commands
into named functions.
(indent-rigidly-left, indent-rigidly-right)
(indent-rigidly-left-to-tab-stop)
(indent-rigidly-right-to-tab-stop): New functions.  Decide on
indentation direction based on bidi direction, and accumulate
sequential commands in a single undo boundary.
(indent-rigidly--pop-undo): New utility function.

* doc/emacs/indent.texi (Indentation Commands): Document C-x TAB changes.

* doc/lispref/text.texi (Region Indent): Note the new interactive
behavior of indent-rigidly.