'libxml-parse(html|xml)-region': new optional param 'discard-comments'.

* doc/lispref/text.texi (Parsing HTML/XML): Document new optional parameter
'discard-comments' of 'libxml-parse(html|xml)-region'.

* src/xml.c (parse_region): Take care of new optional parameter
'discard-comments' of 'libxml-parse(html|xml)-region'.
(Flibxml_parse_html_region, Flibxml_parse_xml_region): New
optional parameter 'discard-comments'.

* test/automated/libxml-tests.el
(libxml-tests--data-comments-preserved): Renamed from
'libxml-tests--data'.
(libxml-tests--data-comments-discarded): New.
(libxml-tests): Check whether 'libxml-parse-xml-region' is
discarding comments correctly.