runtime: document sigtrampgo better

Add an explanation of why sigtrampgo is nosplit.

Updates #21314.

Change-Id: I3f5909d2b2c180f9fa74d53df13e501826fd4316
Reviewed-on: https://go-review.googlesource.com/79615Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/signal_unix.go

@@ -275,6 +275,12 @@ func sigpipe() {
 // sigtrampgo is called from the signal handler function, sigtramp,
 // written in assembly code.
 // This is called by the signal handler, and the world may be stopped.
+//
+// It must be nosplit because getg() is still the G that was running
+// (if any) when the signal was delivered, but it's (usually) called
+// on the gsignal stack. Until this switches the G to gsignal, the
+// stack bounds check won't work.
+//
 //go:nosplit
 //go:nowritebarrierrec
 func sigtrampgo(sig uint32, info *siginfo, ctx unsafe.Pointer) {