[SR-9330] DispatchSourceTimer schedule methods should document the default values

Issue Description:

The DispatchSourceTimer suite of schedule methods all have documentation comments, but the documentation comments don't explain what the default values for parameters are. So for example, the schedule(deadline:repeating:leeway🙂 method does not make it clear whether omitting the repeating: argument will produce a oneshot timer or not. Of course, in this instance if it produced a repeating timer there's no obvious value for what the repeating interval would be, but still, any time I use this method I always have to double-check what the default value is in the source in order to satisfy myself that I really am producing a oneshot timer.

Start by locating the DispatchSourceTimer schedule methods and reading their existing documentation comments in the libdispatch sources. Document the default values for the schedule parameters, including whether omitting repeating creates a one-shot timer, and verify that all schedule methods describe their defaults consistently.