Document when XmlNode.SelectNodes() can return null

Intellisense warns me `XmlNode.SelectNodes()` may return null. I was interested in what circumstances that happens in - but there doesn't seem to be any mention of this method returning null in the docs.

Google seems to suggest it can happen in various unusual situations, but the information seems to be sparse. ([link](https://social.msdn.microsoft.com/Forums/en-US/ae0e751f-5220-4bde-8c3c-0f507403159b/selectnodes-return-null?forum=xmlandnetfx)).

Could this be documented? (If it's not instead an Intellisense error instead, of course!)