runtime.Callers: make documentation match code

It is a bug that Caller and Callers disagree about the offset of the skip
parameter. Document the bug.

R=rsc, dsymonds, r, iant
CC=golang-dev
https://golang.org/cl/5976064

src/pkg/runtime/extern.go

@@ -20,7 +20,8 @@ func Goexit()
 
 // Caller reports file and line number information about function invocations on
 // the calling goroutine's stack.  The argument skip is the number of stack frames
-// to ascend, with 0 identifying the caller of Caller.  The return values report the
+// to ascend, with 1 identifying the caller of Caller.  (For historical reasons the
+// meaning of skip differs between Caller and Callers.) The return values report the
 // program counter, file name, and line number within the file of the corresponding
 // call.  The boolean ok is false if it was not possible to recover the information.
 func Caller(skip int) (pc uintptr, file string, line int, ok bool)