Docs: align multitenancy guide with warehouse replica-limit contract

The multitenancy guide stated the number of child services in a
warehouse is "limited to a small number", contradicting the usage limits
page (no separate services cap; bounded by the combined warehouse replica
limit). The old services-per-warehouse cap was removed, and the linked
warehouse limitations section no longer documents it. Update the guide to
match, pointing to the usage limits page and warehouse scaling section.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>