cmd/internal/obj/x86: fix doc-comment and clarify output parameters

Fixes comment style.
Added names to return parameters to clarify what they are.

Change-Id: Ib6f7f58b36981e08b63de1a4cd582ce99cfb4519
Reviewed-on: https://go-review.googlesource.com/107235Reviewed-by: Iskander Sharipov <iskander.sharipov@intel.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/x86/asm6.go

@@ -5032,7 +5032,7 @@ func (ab *AsmBuf) asmins(ctxt *obj.Link, cursym *obj.LSym, p *obj.Prog) {
 	}
 }
 
-// Extract 4 operands from p.
-func unpackOps4(p *obj.Prog) (*obj.Addr, *obj.Addr, *obj.Addr, *obj.Addr) {
+// unpackOps4 extracts 4 operands from p.
+func unpackOps4(p *obj.Prog) (arg0, arg1, arg2, dst *obj.Addr) {
 	return &p.From, &p.RestArgs[0], &p.RestArgs[1], &p.To
 }