Improve documentation of Ediff wordwise commands

Improve documentation of Ediff wordwise commands

* lisp/vc/ediff.el (ediff-windows-wordwise)
(ediff-windows-linewise, ediff-regions-wordwise): Update and
clarify the doc strings.

* doc/misc/ediff.texi (Major Entry Points): Update and clarify
the documentation of 'ediff-windows-wordwise' and
'ediff-regions-wordwise'.  See the discussion starting at
https://lists.gnu.org/archive/html/help-gnu-emacs/2018-11/msg00197.html
for the details.