Simplify secret-key encryption table and description

Attempt to simplify the secret-key encryption table by listing S2K usage octet values only once, and indicating the differences between v4 and v6 keys where applicable. Also, restructure the accompanying description slightly.

Finally, more explicitly require rejecting v6 keys with legacy secret key encryption formats.

This MR builds on top of !288 (merged), and is intended to be merged into that MR, if this seems reasonable :)