os/exec: document that cmd.Start() sets the Process field

Change-Id: I4f41b680741e9bd2a4e8c094ecf3ce6226e48d12
GitHub-Last-Rev: 8f58bc6c4398cf739b33f8b5368926d6650059c3
GitHub-Pull-Request: golang/go#35934
Reviewed-on: https://go-review.googlesource.com/c/go/+/209558Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/exec/exec.go

@@ -369,6 +369,8 @@ func lookExtensions(path, dir string) (string, error) {
 
 // Start starts the specified command but does not wait for it to complete.
 //
+// If Start returns successfully, the c.Process field will be set.
+//
 // The Wait method will return the exit code and release associated resources
 // once the command exits.
 func (c *Cmd) Start() error {