os/signal: remove comments about SA_RESTART

It's no longer necessary for non-Go signal handlers to use SA_RESTART.

For #20400
Fixes #44761

Change-Id: Ie3c7fecc631a4a2822331754296ea09b308e1391
Reviewed-on: https://go-review.googlesource.com/c/go/+/298269
Trust: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: ianlancetaylor

src/os/signal/doc.go

@@ -129,9 +129,7 @@ If the non-Go code installs any signal handlers, it must use the
 SA_ONSTACK flag with sigaction. Failing to do so is likely to cause
 the program to crash if the signal is received. Go programs routinely
 run with a limited stack, and therefore set up an alternate signal
-stack. Also, the Go standard library expects that any signal handlers
-will use the SA_RESTART flag. Failing to do so may cause some library
-calls to return "interrupted system call" errors.
+stack.
 
 If the non-Go code installs a signal handler for any of the
 synchronous signals (SIGBUS, SIGFPE, SIGSEGV), then it should record