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.

Clarify the content type requirement (for message formats).

This reverts commit 79759fea, reversing
changes made to dc36f4df.

Without the space they do not display correctly.

Clarify the use of U-A header.