Support XHTML in client catalog documentation element

[cwardgar]

From the THREDDS mailing list.

When a server catalog documentation element includes XHTML content, that XHTML is not passed along to the generated client catalog as one would expect. For example, if the server catalog contains:

<documentation xmlns:xhtml="https://www.w3.org/1999/xhtml">
    I have a <xhtml:b>cool</xhtml:b> link!
    I call it <xhtml:a href="https://www.met.no">Meteorologisk Institutt</xhtml:a>!
</documentation>

The client catalog version will contain:

<documentation>I have a link! I call it !</documentation>

All of the XHTML elements are stripped out! Indeed, it seems that any "unrecognized" element is stripped, although I'm not exactly sure how the JDOM parser determines that, as it's not validating the catalog.

We'd like those XHTML elements to be preserved, so that when the HTML version of the client catalog is generated, that content will be displayed on the page with the expected formatting.