debug/dwarf: address TODO in doc comment for New

Fixes #2844.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5675072

src/pkg/debug/dwarf/open.go

@@ -31,8 +31,9 @@ type Data struct {
 }
 
 // New returns a new Data object initialized from the given parameters.
-// Clients should typically use [TODO(rsc): method to be named later] instead of calling
-// New directly.
+// Rather than calling this function directly, clients should typically use
+// the DWARF method of the File type of the appropriate package debug/elf,
+// debug/macho, or debug/pe.
 //
 // The []byte arguments are the data from the corresponding debug section
 // in the object file; for example, for an ELF object, abbrev is the contents of