os: document inheritance of thread state over exec

Fixes #23570.

Change-Id: I462ada2960d710c2c94dc22a59d292703d83f612
Reviewed-on: https://go-review.googlesource.com/90255Reviewed-by: Russ Cox <rsc@golang.org>

src/os/exec.go

@@ -88,6 +88,11 @@ func FindProcess(pid int) (*Process, error) {
 // specified by name, argv and attr. The argv slice will become os.Args in the
 // new process, so it normally starts with the program name.
 //
+// If the calling goroutine has locked the operating system thread
+// with runtime.LockOSThread and modified any inheritable OS-level
+// thread state (for example, Linux or Plan 9 name spaces), the new
+// process will inherit the caller's thread state.
+//
 // StartProcess is a low-level interface. The os/exec package provides
 // higher-level interfaces.
 //

src/os/exec/exec.go

@@ -293,6 +293,11 @@ func (c *Cmd) closeDescriptors(closers []io.Closer) {
 //
 // If the command starts but does not complete successfully, the error is of
 // type *ExitError. Other error types may be returned for other situations.
+//
+// If the calling goroutine has locked the operating system thread
+// with runtime.LockOSThread and modified any inheritable OS-level
+// thread state (for example, Linux or Plan 9 name spaces), the new
+// process will inherit the caller's thread state.
 func (c *Cmd) Run() error {
 	if err := c.Start(); err != nil {
 		return err