Fix nxml-get-inside (Bug#32003)

Fix nxml-get-inside (Bug#32003)

The change from 2016-01-16 "lisp/nxml: Use syntax-tables for comments"
made nxml-get-inside return non-nil for any string or comment,
including attribute strings.  This caused incorrect and therefore
indentation.
* lisp/nxml/nxml-rap.el: Update commentary to reflect changes to
nxml-mode parsing.
(nxml-get-inside): Only return non-nil when inside comments and
generic strings, not normal quote-delimited strings.
* test/lisp/nxml/nxml-mode-tests.el: New tests.