syscall: document LockOSThread with GNU/Linux SysProcAttr.Ptrace

Fixes #28315

Change-Id: Ie02c72d02ad2f66c9cdbbba579a304641f327672
Reviewed-on: https://go-review.googlesource.com/c/155138Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/exec_linux.go

@@ -20,9 +20,12 @@ type SysProcIDMap struct {
 }
 
 type SysProcAttr struct {
-	Chroot       string         // Chroot.
-	Credential   *Credential    // Credential.
-	Ptrace       bool           // Enable tracing.
+	Chroot     string      // Chroot.
+	Credential *Credential // Credential.
+	// Ptrace tells the child to call ptrace(PTRACE_TRACEME).
+	// Call runtime.LockOSThread before starting a process with this set,
+	// and don't call UnlockOSThread until done with PtraceSyscall calls.
+	Ptrace       bool
 	Setsid       bool           // Create session.
 	Setpgid      bool           // Set process group ID to Pgid, or, if Pgid == 0, to new pid.
 	Setctty      bool           // Set controlling terminal to fd Ctty (only meaningful if Setsid is set)