Fix #77040: tidyNode::isHtml() is completely broken

The documentation of `tidyNode::isHtml()` states that this method
"checks if a node is part of a HTML document". That is, of course,

Fix #77040: tidyNode::isHtml() is completely broken

The documentation of `tidyNode::isHtml()` states that this method
"checks if a node is part of a HTML document". That is, of course,
nonsense, since a tidyNode is "an HTML node in an HTML file, as
detected by tidy."

What this method is actually supposed to do is to check whether a node
is an element (unless it is the root element). This has been broken by
commit d8eeb8e[1], which assumed that `enum TidyNodeType` would
represent flags of a bitmask, what it does not.

[1] <http://git.php.net/?p=php-src.git;a=commit;h=d8eeb8e28673236bca3f066ded75037a5bdf6378>

Closes GH-6290.

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