Document certificate verification functions in SSL.

Or at least group them together and make a passing attempt to document
them. The legacy X.509 stack itself remains largely untouched and most
of the parameters have to do with it.

Change-Id: I9e11e2ad1bbeef53478c787344398c0d8d1b3876
Reviewed-on: https://boringssl-review.googlesource.com/5942
Reviewed-by: Adam Langley <agl@google.com>