runtime: remove debugging knob to turn off preemption

It's still easy to turn off, but the builders are happy.
Also document.

R=golang-dev, iant, dvyukov
CC=golang-dev
https://golang.org/cl/12371043

doc/go1.2.txt

@@ -26,6 +26,7 @@ image/draw: added Quantizer type (CL 11148043).
 image/gif: added Encode and EncodeAll (CL 10896043).
 io: Copy prioritizes WriterTo over ReaderFrom (CL 9462044).
 net: new build tag netgo for building a pure Go net package (CL 7100050).
+runtime: preemption of goroutines at function entry (CL 12371043).
 sort: new Stable function provides stable sort (CL 9612044).
 syscall: implemented Sendfile for Darwin, added Syscall9 for Darwin/amd64 (CL 10980043).
 testing: AllocsPerRun is now quantized to an integer (the type is still float64) (CL 9837049).

src/pkg/runtime/proc.c

@@ -2275,12 +2275,6 @@ preemptone(P *p)
 	M *mp;
 	G *gp;
 
-// Preemption requires more robust traceback routines.
-// For now, disable.
-// The if(1) silences a compiler warning about the rest of the
-// function being unreachable.
-if(0) return;
-
 	mp = p->m;
 	if(mp == nil || mp == m)
 		return;