os/signal: document behavior of SIGPIPE on non-Go thread

Fixes golang#56150

Change-Id: Id990783562950ba8be7ce9526b7a811625f2190a
Reviewed-on: https://go-review.googlesource.com/c/go/+/442415
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>

Author: ianlancetaylor

src/os/signal/doc.go

@@ -164,6 +164,12 @@ signal, and raises it again, to invoke any non-Go handler or default
 system handler. If the program does not exit, the Go handler then
 reinstalls itself and continues execution of the program.
 
+If a SIGPIPE signal is received, the Go program will invoke the
+special handling described above if the SIGPIPE is received on a Go
+thread.  If the SIGPIPE is received on a non-Go thread the signal will
+be forwarded to the non-Go handler, if any; if there is none the
+default system handler will cause the program to terminate.
+
 # Non-Go programs that call Go code
 
 When Go code is built with options like -buildmode=c-shared, it will