Missing description of `uxpContainer` in `HTMLWebViewElement.md`

https://github.com/AdobeDocs/uxp/blob/47510097aaf62876dfe00a01e00461ce5018e6cc/src/pages/uxp/reference-js/Global%20Members/HTML%20Elements/HTMLWebViewElement.md?plain=1#L189-L192

Since UXP-specific, without a description, exposing this to 3P doesn't make a lot of sense. Don't get me wrong: I think if it makes sense we should expose this to 3P, but exposing it without any information doesn't add a lot of value.

_Originally posted by @pklaschka in https://github.com/AdobeDocs/uxp/pull/61#discussion_r997125340_

Open src/pages/uxp/reference-js/Global Members/HTML Elements/HTMLWebViewElement.md and start around lines 189-192, where uxpContainer is listed. Add a description explaining the property’s UXP-specific behavior; the issue is done when the entry provides useful information for third-party developers.

Written by the indexing model from the issue text.