Improve documentation of kill commands

Improve documentation of kill commands

* lisp/simple.el (region-extract-function, delete-backward-char)
(delete-forward-char, kill-region, copy-region-as-kill)
(kill-ring-save): Better document the optional argument REGION in
the doc strings.  Mention in the doc strings that text put in the
kill-ring can be filtered by 'filter-buffer-substring'.

* doc/lispref/text.texi (Kill Functions): Mention that functions
described in this subsection can filter text they put in the
kill-ring.  Add a cross-reference to "Buffer Contents" and an
index entry.  Document the optional argument 'region' and its
effect.
(Bug#21315)