add mention of tag string to grammar comment

R=rsc
DELTA=3  (2 added, 0 deleted, 1 changed)
OCL=18232
CL=18232

src/lib/reflect/type.go

@@ -452,10 +452,12 @@ func init() {
 		functiontype
 	typename =
 		name '.' name
+	doublequotedstring = 
+		string in " ";  escapes are \0 (NUL) \n \t \" \\
 	fieldlist =
 		[ field { [ ',' | ';' ] field } ]
 	field =
-		identifier stubtype
+		identifier stubtype [ doublequotedstring ]
 	arraytype =
 		'[' [ number ] ']' stubtype
 	structtype =