chore: increasing `cpu.limits` for `coderd` and comments for `replicas` and `coderd` cpu mem adjustment guidance

[sharkymark]

These recommendations are to better serve Coder deployments with more users.

1. coderd's CPU limit was the same as the request 250m, which is low. Expanding the CPU limit to 500.
2. added comments to coderd.replicas to point out that counts should increase as more nodes and users are added to a deployment