Ponyc: some stdlib packages do not have package documentation in a file with the same name as the package

[aturley]

The convention in the Pony standard library is to include package documentation in a file called PACKAGE.pony, where PACKAGE is the name of the package. This makes it easy for someone to find the file that contains the package-level documentation.

The following packages have their documentation in other files:

• bureaucracy -- documentation in custodian.pony
• ponybench -- documentation in pony_bench.pony (is this close enough?)
• process -- documentation in process_monitor.pony
• promises -- documentation in promise.pony (is this close enough?)
• signals -- documentation in sig.pony

[aturley]

There's currently no rule in the style guide for this. I think we should update the style guide to say that packages should contain a file called PACKAGE.pony with the package-level documentation, and then update the packages accordingly. Any thoughts?