docs(pods): updates description of config for tmpDirSizeLimit

[PaulRMellor]

Documentation

Updates property description of tmpDirSizeLimit for PodTemplate schema.
Clarifies that size is allocated is from pod memory

Checklist

Please go through this checklist and make sure all applicable tasks have been done

• Write tests
• Make sure all tests pass
• Update documentation
• Check RBAC rights for Kubernetes / OpenShift roles
• Try your changes from Pod inside your Kubernetes and OpenShift cluster, not just locally
• Reference relevant issue(s) and close them after merging
• Update CHANGELOG.md
• Supply screenshots for visual changes, such as Grafana dashboards