Markdown.pl: clarify help text for --no-validate-xml option

Replace the old text left over from before --validate-xml-internal
existed with something more accurate that indicates that Markdown.pl
can check for XML problems itself (via the default --validate-xml-internal
option).

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>

Markdown.pl

@@ -4758,9 +4758,9 @@ used).  However, any raw tags in the input (that are on the "approved"
 list), could potentially result in invalid XML output (i.e. mismatched
 start and end tags, missing start or end tag etc.).
 
-Markdown.pl will I<NOT check> for these issues itself.  But with
+Markdown.pl can check for these issues itself using its own internal
-the B<--validate-xml> option will use C<XML::Simple> or C<XML::Parser>
+B<--validate-xml-internal> check or, with the B<--validate-xml>
-to do so.
+option, it can use C<XML::Simple> or C<XML::Parser> to do so.
 
 Note that B<--validate-xml-internal> is the default option unless
 B<--no-sanitize> is used in which case B<--no-validate-xml> is the