Docs should be clearer about watch option keys not supporting key paths

[DanielAsher]

Version

3.0.0-rc.9

Reproduction link

https://jsfiddle.net/oekwvhd4/

Steps to reproduce

Migrate a v2 app to v3 with a component that uses a watch option with a key path.

What is expected?

The docs clearly state that watch options no longer support key expressions.

Clear migration guidance should be given about moving watched keypaths to mounted this.$watch(() => this.key.path, () => doSomething())

What is actually happening?

This is a significant (and silent!) breaking change, but the docs still refer to key expressions.

[Ydot19]

If this is a documentation issue, I feel it's best for you to report at docs-next because that's the repo that deals with the migration guide

[DanielAsher]

done

thanks for the awesome work!

[CyberAP]

It's strange that it didn't go through an RFC process. I can understand removing it from the $watch function, but removing it from the watch option doesn't look like an improvement to me (you'll have to create a computed for that watcher to work now).

[skirtles-code]

This was mentioned briefly in #671 but it looks like it got lost in the subsequent discussion about potential changes to immediate.