Revise documentation for inter-frame messages.

Revise the documentation in order to:

 - Reduce uninformative noise ("the X frame informs the Y frame to..."). The
   source and destination frame type are given by the type name.

 - Re-phrase the documentation to read more like either a function description
   or an event description, depending on whether the message is a specific
   command/request or a more general event.

 - Use the plural "guests" to make it clear that there can be more than one
   guest frame.

 - Clarify when events specific to third-party authorities are used.