Mark `t.i.process.Process` and `t.i.process.PTYProcess` as implementing IProcessTransport
|Reported by:||Screwtape||Owned by:|
branch-diff, diff-cov, branch-cov, buildbot
Currently the IProcessTransport documentation claims that the only process transport in Twisted is for Win32, which is a total lie.
t.i.process.PTYProcess also implement it, and are very likely much more solid implementations.
We should mark both those classes as implementing that interface.
(this was filed once before as #4479, but that ticket got derailed into a different documentation issue)