Consolidate 'widen' calls

Consolidate 'widen' calls

* lisp/progmodes/prog-mode.el (prog-indentation-context):
Un-document all elements but the first.
(prog-widen): Remove.
(http://lists.gnu.org/archive/html/emacs-devel/2017-12/msg00321.html)

* doc/lispref/text.texi (Mode-Specific Indent): Update.

* lisp/progmodes/ruby-mode.el (ruby-calculate-indent):
Don't call widen.

* lisp/progmodes/python.el
(python-indent-guess-indent-offset)
(python-info-current-defun): Replace prog-widen with widen;
these functions are not called during indentation.
(python-indent-context)
(python-indent--calculate-indentation)
(python-info-dedenter-opening-block-message)
(python-info-line-ends-backslash-p)
(python-info-beginning-of-backslash)
(python-info-continuation-line-p)
(python-info-current-defun): Remove 'widen' calls.

* lisp/indent.el (indent-according-to-mode)
(indent-for-tab-command, indent-region): Move them here.

* lisp/textmodes/mhtml-mode.el (mhtml-indent-line):
Bind prog-indentation-context to one-element list.