os: document that FindProcess always succeeds on Unix

Fixes #14146

Change-Id: I892ca4ccdc1ba785750e1eae800852dc5825156c
Reviewed-on: https://go-review.googlesource.com/19093
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/os/doc.go

@@ -7,9 +7,13 @@ package os
 import "time"
 
 // FindProcess looks for a running process by its pid.
+//
 // The Process it returns can be used to obtain information
 // about the underlying operating system process.
-func FindProcess(pid int) (p *Process, err error) {
+//
+// On Unix systems, FindProcess always succeeds and returns a Process
+// for the given pid, regardless of whether the process exists.
+func FindProcess(pid int) (*Process, error) {
 	return findProcess(pid)
 }