Document preauth flags for service principals

These flags are overloaded to mean different things for clients and
servers; previously we only documented the client behavior.

(cherry picked from commit 7425e9b69566c241c54eb2686fb37f216122423f)

ticket: 7653
version_fixed: 1.11.3
status: resolved
Clarify retiring-des based on user feedback

Explain why DES keys should be removed from principals, and clarify
that allow_weak_crypto overrides all other configuration.

(cherry picked from commit fa6de1bf73926751a2f68bff31ef020eb7db9260)

ticket: 7654
version_fixed: 1.11.3
status: resolved
Clean up dangling antecedent in allow_weak_crypto

The "previous three lists" are not previous any more.
Say explicitly which three lists, and make the parenthetical bind
to the correct noun.

(cherry picked from commit 2a10e19e19c65af0e3890bdeae03c37089ef02ea)

ticket: 7655
version_fixed: 1.11.3
status: resolved