Motivation:
`Http2FrameCodecBuilder` defines static factory methods `forClient()`
and `forServer()` that should be used to create a new instance.
The default ctor is useful only when users need to override behavior
of the existing builder. Those users should define another way to create
an instance.
Modifications:
- Decrease visibility of `Http2FrameCodecBuilder` default ctor from
`public` to `protected`;
- Add javadoc to clarity responsibilities;
Result:
Users of `Http2FrameCodecBuilder` are not confused why
`new Http2FrameCodecBuilder().build()` works for the server-side, but
does not work for the client-side.
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