As the number of supported encryption modes has grown, the part of the
"Encryption modes and usage" section that describes the supported
encryption modes has gotten a bit messy.  It presents useful
information, but it's a bit lacking in high-level context.

Rework the section to hopefully be much more useful.

Link: https://lore.kernel.org/r/20230630064811.22569-2-ebiggers@kernel.orgSigned-off-by: Eric Biggers <ebiggers@google.com>