doc: Add comments for socket descriptor handling when forking #108

I find logic around forking child processes always a bit confusing, hopefully these comments can make it a bit easier for a future reader.

Code review ACK b56bf218c7d5e2666dbc6b85900af113181260dc

Separately, it looks like there is also a potential leak in that function because both socketpair descriptors will be leaked if fork() fails. But probably this is not a serious issue because if fork() fails, something is already very wrong and the process might be to exit anyway.