Docs: remove per-service 20-replica cap from API overview

Drop the hard "between 2 and 20" upper bound for numReplicas from the API
overview, keeping the minimum-replica guidance and pointing to the usage
limits page for the ceiling, consistent with moving replica limits to the
warehouse level.

Note: the Cloud OpenAPI spec (docs/_specs/cloud-openapi.json) still
encodes maximum: 20 for numReplicas/minReplicas/maxReplicas, which the
generated API reference pages render. That file is robot-synced from the
upstream OpenAPI source and must be updated there, not in this repo.

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