ZoneSpecification and related types are underdocumented

[Hixie]

I was trying to figure out how to catch all errors (sync and async) thrown while running a function. I figured out I probably want Zone.current.fork().runGuarded(), but when trying to figure out the arguments to fork I found that the following APIs are undocumented or underdocumented:

• ZoneSpecification constructors don't explain their arguments
• ZoneSpecification properties have no documentation
• The typedefs used in those properties, such as CreatePeriodicTimerHandler, CreateTimerHandler, ErrorCallbackHandler, etc, have no documentation
• The class itself doesn't say what the difference between errorCallback and handleUncaughtError is

See also #26455, #26454, #17667.