Documentation: Add note about yjs property syncing

[etrepum]

lexical-collab will only reliably sync enumerable properties of nodes. This means that nodes participating in collab that are using properties instead of NodeState should initialize all of their properties in the constructor, even if to undefined.

We should probably also do a sweep over the existing nodes to make sure they all have explicit initializers (maybe write a lint to check for optional properties in node subclasses and disallow that syntax, foo: undefined | string will require explicit constructor initialization when foo?: string does not)

See also #7276, #7277