trace: update the documentation

This commit adds some missing symbols and other minor enhancements.
In particular, it establishes the term 'channel' as a synonym for
a BIO object attached to a trace category, and introduces the
concept of a 'simple' channel versus a 'callback' channel.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8463)