docs: document the session-drain-soonest strict mode

The strategy docs describe only the sticky drain-soonest variant, so the
new strict mode is invisible to anyone configuring lb_strategy, and the
sticky variant's most surprising production property goes unmentioned.

A reader picking a strategy from configuration.md or tracing a routing
decision through routing-architecture.md finds no session-drain-soonest-strict
entry and no explanation of why sticky mode concentrates all traffic on the
most recently started session.

Now both documents list the strict variant with its comparator chain
(weekly reset, active-session tie-break, priority, utilization, account
id), configuration.md carries the multi-active-session caveat of the
sticky mode, and the hardcoded four-strategy counts are neutral so the
lists cannot drift again.