os/exec: document ProcessState available after a call to Wait or Run

os/exec: document ProcessState available after a call to Wait or Run

Wait or Run will populate its ProcessState when the command completes.

Fixes #56002.

Change-Id: I21547431f5d2d3e0fc0734fd1705421a0ac4209c
Reviewed-on: https://go-review.googlesource.com/c/go/+/437996
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>