Clarify lockout replication issues in docs

In the "KDC replication and account lockout" section of lockout.rst,
specifically call out kprop and incremental propagation as the
mechanisms which do not replicate account lockout state, and add a
note that KDCs using LDAP may not be affected by that section's
concerns.

(cherry picked from commit 8eb9e6fe1b01faa875dcf91b618ad4cd7793438a)

ticket: 7773
version_fixed: 1.12
status: resolved