Remove the section for b64 encoded trailers. As gRPC-Web uses CRLF to separate the trailers in the trailers frame, a binary trailer need to reserve in a base64 encoded format for their values. This cannot be skipped even base64 is applied on the whole trailers frame per client's request via content-type: application/grpc-web-text.
1. Clarify about trailer-only responses
2. Switch to http/1.1 header block format for in-body trailer encoding
3. Clarify how compatibility is supported (using U-A and Server headers respectively)