go.tools/go/types: documentation follow-up

Added documentation per request in
https://golang.org/cl/108230044 .

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/137620044

go/types/type.go

@@ -200,7 +200,7 @@ type Signature struct {
 	recv     *Var   // nil if not a method
 	params   *Tuple // (incoming) parameters from left to right; or nil
 	results  *Tuple // (outgoing) results from left to right; or nil
-	variadic bool   // true if the last parameter's type is of the form ...T
+	variadic bool   // true if the last parameter's type is of the form ...T (or string, for append built-in only)
 }
 
 // NewSignature returns a new function type for the given receiver, parameters,

go/types/typestring.go

@@ -221,6 +221,11 @@ func writeTuple(buf *bytes.Buffer, this *Package, tup *Tuple, variadic bool, vis
 					buf.WriteString("...")
 					typ = s.elem
 				} else {
+					// special case:
+					// append(s, "foo"...) leads to signature func([]byte, string...)
+					if t, ok := typ.Underlying().(*Basic); !ok || t.kind != String {
+						panic("internal error: string type expected")
+					}
 					writeType(buf, this, typ, visited)
 					buf.WriteString("...")
 					continue