Start generating documentation for ssl.h.

There's this giant "Underdocumented functions" section in the middle, but it
doesn't look too silly once the "Deprecated methods" section is merged in with
the other deprecated functions.

Change-Id: Ib97d88b0f915f60e9790264474a9e4aa3e115382
Reviewed-on: https://boringssl-review.googlesource.com/4291
Reviewed-by: Adam Langley <agl@google.com>