runtime: document that Caller and Frame.File always use forward slashes

Document that Caller and Frame.File always use forward slashes
as path separators, even on Windows.

Fixes #3335

Change-Id: Ic5bbf8a1f14af64277dca4783176cd8f70726b91
Reviewed-on: https://go-review.googlesource.com/c/go/+/603275
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

src/runtime/extern.go

@@ -294,10 +294,11 @@ import (
 
 // 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.  (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.
+// 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, the file name (using forward slashes as path separator, even
+// on Windows), and the 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) {
 	rpc := make([]uintptr, 1)
 	n := callers(skip+1, rpc)

src/runtime/symtab.go

@@ -49,7 +49,8 @@ type Frame struct {
 	// File and Line are the file name and line number of the
 	// location in this frame. For non-leaf frames, this will be
 	// the location of a call. These may be the empty string and
-	// zero, respectively, if not known.
+	// zero, respectively, if not known. The file name uses
+	// forward slashes, even on Windows.
 	File string
 	Line int