[Feature] Add docs for @updateURL

**Motivation**
Developers need proper docs in order to use all features supported by violentmonkey.
Seems like there is no info in the https://violentmonkey.github.io/api/metadata-block/ about `@updateURL`

**Proposed Solution**
1. Explain if `@updateURL` is supported or not. An can be used for *.meta.js files only.
2. Give example of usage at public docs.


