runtime: fix docs for Caller and Callers

The previous attempt to explain this got it backwards (all the more reason to be
sad we couldn't make the two functions behave the same).

Fixes #3669.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6249051

src/pkg/runtime/extern.go

@@ -20,7 +20,7 @@ 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 1 identifying the caller of Caller.  (For historical reasons the
+// to ascend, with 0 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.
@@ -28,7 +28,8 @@ func Caller(skip int) (pc uintptr, file string, line int, ok bool)
 
 // Callers fills the slice pc with the program counters of function invocations
 // on the calling goroutine's stack.  The argument skip is the number of stack frames
-// to skip before recording in pc, with 0 starting at the caller of Callers.
+// to skip before recording in pc, with 0 identifying the frame for Callers itself and
+// 1 identifying the caller of Callers.
 // It returns the number of entries written to pc.
 func Callers(skip int, pc []uintptr) int