debug/gosym: correct comments for Table.{Files,Objs}

The fields aren't too useful for Go 1.2 and later, but they aren't
actually nil.

Fixes #38754

Change-Id: Ia13a224f623697a00dea8ba0225633e1b9308c9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/230940
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/debug/gosym/symtab.go

@@ -121,8 +121,8 @@ type Obj struct {
 type Table struct {
 	Syms  []Sym // nil for Go 1.3 and later binaries
 	Funcs []Func
-	Files map[string]*Obj // nil for Go 1.2 and later binaries
-	Objs  []Obj           // nil for Go 1.2 and later binaries
+	Files map[string]*Obj // for Go 1.2 and later all files map to one Obj
+	Objs  []Obj           // for Go 1.2 and later only one Obj in slice
 
 	go12line *LineTable // Go 1.2 line number table
 }