os: document the Process.Pid field

[perillo]

As discussed in https://groups.google.com/forum/#!topic/golang-nuts/ievQGPz1R7k, https://golang.org/cl/4029053 dropped the documentation for the Pid field.

I also propose to extend the documentation with something like:

The value of Pid after the process has been waited/released is not specified.

The reason is that on UNIX systems, Release set Pid to -1, but not on Windows.
This means that a program can not check if Pid == -1 to determine if the process has been released.

The alternative is to change the Windows implementation.

[ianlancetaylor]

Seems like the first step is to document the Pid field at all.

[tiriplicamihai]

I can help with this. Will have a PR ready today.

[gopherbot]

Change https://golang.org/cl/217105 mentions this issue: os: document Pid field from Process