runtime: remove the 'go:nosplit' directive from documentation

The //go:nosplit directive was visible in GoDoc because the function
that it preceeded (Gosched) is exported. This change moves the directive
above the documentation, hiding it from the output.

Change-Id: I281fd7573f11d977487809f74c9cc16b2af0dc88
Reviewed-on: https://go-review.googlesource.com/69120
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/proc.go

@@ -249,9 +249,10 @@ func forcegchelper() {
 	}
 }
 
+//go:nosplit
+
 // Gosched yields the processor, allowing other goroutines to run. It does not
 // suspend the current goroutine, so execution resumes automatically.
-//go:nosplit
 func Gosched() {
 	mcall(gosched_m)
 }