DOC: Add note about encrypted password CPU usage

DOC: Add note about encrypted password CPU usage

From first-hand experience I realized that using encrypted passwords in
userlists can quickly become overwhelming for busy sites. In my case
just about 100 rq/s were enough to drive (user) CPU usage from 2-3% up
to >90%. While it is perfectly explicable why this is the case, having
it mentioned in the relevant documentation section might spare someone
some confusion in the future.