document wire.Struct first argument (#197)

wire.go

@@ -146,9 +146,11 @@ func InterfaceValue(typ interface{}, x interface{}) ProvidedValue {
 // A StructProvider represents a named struct.
 type StructProvider struct{}
 
-// Struct specifies that the given struct type will be provided by filling in the fields
-// in the struct that have the names given. Each of the arguments must be a name
-// to the field they wish to reference. As a special case, if a single name "*"
+// Struct specifies that the given struct type will be provided by filling in the fields in the struct that have the names given.
+//
+// The first argument must be a pointer to the struct type. For a struct type
+// Foo, Wire will use field-filling to provide both Foo and *Foo. The remaining
+// arguments are field names to fill in. As a special case, if a single name "*"
 // is given, then all of the fields in the struct will be filled in.
 //
 // For example: