Support proto2 default values and field presence (#716)

docs/generated_code.md

@@ -134,8 +134,8 @@ foo = "";
 bar?: string;
 ```
 
-Note that all scalar fields have an intrinsic default value in proto3 syntax, unless they are marked
-as `optional`. Protobuf types map to ECMAScript types as follows:
+Note that all scalar fields have a zero-value in proto3 syntax, unless they are 
+marked as `optional`. Protobuf types map to ECMAScript types as follows:
 
 | protobuf type | ECMAScript type | default value        |
 |---------------|-----------------|----------------------|

docs/runtime_api.md

@@ -68,10 +68,9 @@ You can create an instance with the `new` keyword:
 const user = new User();
 ```
 
-
 For convenience, constructors accept an initializer object. All fields in the 
-initializer object are optional, and if not provided, the default value for the 
-field is used.
+initializer object are optional, and if not provided, the field keeps its 
+default value.
 
 ```typescript
 const user = new User({
@@ -1163,12 +1162,7 @@ let plain: PlainMessage<User> = {
 };
 ```
 
-As such, [`PlainMessage<T>`][src-PlainMessage] can be a great fit to use 
-throughout your business logic, if that business logic is never concerned with 
-serialization, and does not need `instanceof`.
-
-Note that any `T` (assuming `T` extends `Message`) is assignable to a variable 
-of type [`PlainMessage<T>`][src-PlainMessage].
+You can convert any message to a plain message with the function [`toPlainMessage`][src-toPlainMessage].
 
 
 ### AnyMessage

packages/protobuf-bench/README.md

@@ -10,5 +10,5 @@ server would usually do.
 
 | code generator      | bundle size             | minified               | compressed         |
 |---------------------|------------------------:|-----------------------:|-------------------:|
-| protobuf-es         | 97,378 b      | 41,628 b | 10,766 b |
+| protobuf-es         | 98,372 b      | 41,985 b | 10,933 b |
 | protobuf-javascript | 394,384 b  | 288,654 b | 45,122 b |