Ticket 427: Editorial: descriptions of the dynamic features need to be clearer and more consistent

See #426 for background.

• Add a new section 3.6 that includes the definition of 'dynamic extent' currently in section 6.10 and a definition of 'dynamic environment'. Mention that the dynamic environment is captured by call/cc. Say something about threads, if necessary.

• Remove the paragraph about threads from the section on dynamic bindings.

• In the description of dynamic-wind, say that the dynamic environment that is in place when the before and after thunks are called is the the dynamic environment of the call to dynamic-wind.

• 6.11 paragraph 2; change "The system implicitly maintains a current exception handler" to "The system implicitly maintains a current exception handler in the dynamic environment".

• Description of with-exception-handler: change "Handler is installed as the current exception handler for the dynamic extent (as determined by dynamic-wind) of the invocation of thunk" to "Handler is installed as the current exception handler in the dynamic environment used for the invocation of thunk".