Document config flag `hydratedFlag`

[JMDidon]

Hello 🙂

I recently had an issue with the hydrated class that gets added by Stencil to indicate a component is ready to be rendered. Long story short, I have a global MutationObserver that gets added by a 3rd party that I can't modify nor remove/replace, and having this class added for every component can be expensive in this scenarion.

Thanks to the community in Stencil's Slack channel I have eventually found the hydratedFlag in the config that covers my need. But I think this flag should be exposed in the documentation too for people that might need it.

I would be happy to propose a PR if it gets accepted!