syscall: clarify Pdeathsig documentation on Linux

This is a rather large footgun, so let's mention that it sends the signal on thread termination and not process termination in the documentation.

Updates #27505

Change-Id: I489cf7136e34a1a7896067ae24187b0d523d987e
GitHub-Last-Rev: c8722b2
GitHub-Pull-Request: #53365
Reviewed-on: https://go-review.googlesource.com/c/go/+/412114
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: Tasssadar

src/syscall/exec_linux.go

@@ -45,7 +45,11 @@ type SysProcAttr struct {
 	// number in the parent process.
 	Foreground   bool
 	Pgid         int            // Child's process group ID if Setpgid.
-	Pdeathsig    Signal         // Signal that the process will get when its parent dies (Linux and FreeBSD only)
+	// Pdeathsig, if non-zero, is a signal that the kernel will send to
+	// the child process when the creating thread dies. Note that the signal
+	// is sent on thread termination, which may happen before process termination.
+	// There are more details at https://go.dev/issue/27505.
+	Pdeathsig    Signal
 	Cloneflags   uintptr        // Flags for clone calls (Linux only)
 	Unshareflags uintptr        // Flags for unshare calls (Linux only)
 	UidMappings  []SysProcIDMap // User ID mappings for user namespaces.