Document everything in ssl_ciph.c, now ssl_cipher.c.

Just about everything depends on SSL_CIPHER. Move it to the top as the first
section in ssl.h. Match the header order and the source file order and document
everything. Also make a couple of minor style guide tweaks.

Change-Id: I6a810dbe79238278ac480e5ced1447055715a79f
Reviewed-on: https://boringssl-review.googlesource.com/4290
Reviewed-by: Adam Langley <agl@google.com>