[SR-9329] Dispatch overlay desperately needs documentation

[lilyball]

Previous ID	SR-9329
Radar	None
Original Reporter	@lilyball
Type	Bug

Environment

Apple Swift version 4.2.1 (swiftlang-1000.11.42 clang-1000.11.45.1)
Target: x86_64-apple-darwin18.2.0

Additional Detail from JIRA

Votes	0
Component/s	libdispatch
Labels	Bug
Assignee	None
Priority	Medium

md5: 69b16d2b08ab13c98778faa201c655b8

Issue Description:

The Dispatch overlay is almost completely lacking in documentation. This makes it very aggravating to use.

[lilyball]

It turns out that at least some parts of the overlay have doc comments, but the documentation is missing from Apple's Docset. I don't know what the process is for getting Swift overlay documentation into Apple's Docset.

For example, the DispatchSourceTimer schedule methods all have extensive documentation comments, but no documentation at all in Apple's Docset.

[belkadan]

cc @natecook1000