Add remark about reading SharedValue from the js thread

[j-piasecki]

Summary

Reading from a shared value on the js thread may block the JS thread until the UI runtime finishes whatever it's doing. This isn't really communicated clearly and adding it to the docs seems like the best option as we don't want to show warnings.

Test plan

I asked ChatGPT to point out problems (it did).

[joe-sam]

This is confusing 🫡 and possibly requires some investigation. Shouldn't all Read operations on a shared objects be non-blocking unless there are one or more multiple write operation (object locks) on the object in question on the UI thread.