docs: Specify resource_limits shape for CPU quota

[dom-calautti]

… API

Reporter posted "cpu_quota" in the create body for /libpod/containers/create, following the naming of the --cpu-quota CLI flag, and it got silently ignored.

The container was created fine, but the quota was never applied.

There's no top-level cpu_quota field on this endpoint at all. The flat "cpu_quota" field only exists on the separate Docker-compatible /containers/create endpoint, so it's an easy mix-up if you're going off the CLI flag name.

Verified this issue still exists: The flat field is silently dropped, the nested one works as expected (see screenshot below)

Added the correct shape as a comment on ResourceLimits in specgen.go, since that's what the libpod create endpoint's generated Swagger docs are sourced from.

Fixes: #19764

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[mheon]

This should probably go in the API swagger documentation, not here - doesn't make much sense to describe it with JSON payloads in documentation for local usage