document the new parent() Jsoup function in our docs

[thadguidry]

We forgot to update our docs to add the new parent() Jsoup function from #4184 into the /manual/grelfunctions#jsoup-xml-and-html-parsing

Returns the parent node or null if no parent. Use it in conjunction with parseHtml() and select() to provide an element.

[thadguidry]

The parent() function is now showing in the GREL functions on our docs site. Yippee!