(draft) documentation improvements

[yenda]

Just leaving some notes here from our Slack conversations

• invoke element could also include in its docstring the fact that on-exit elements from the child are executed when the state containing invoke is exited (I assume only the on-exit of active states?)
• if a child (invoked statechart) wants to send events to the parent (e/parent-session-id env) gets the parent-session-id and can be used in Send targetexpr

Guidelines for colocated statecharts with UI components:

• Clearly state (via aliases) the things you need to know to properly render the UI
• Write a statechart that produces that data
• In cases where the data in the fulcro state database already represents some “state”, don’t re-model that in the chart (e.g. form dirty is a fact derived from data, not structure)