Add note that --rmm-async only affects distributed scheduler.

[bdice]

Description

The PDS-H queries don't obey --rmm-async unless using the distributed scheduler. This adds a note to the docs.

Perhaps we should solve this differently, so that --rmm-async actually works for all schedulers?

A workaround is setting POLARS_GPU_ENABLE_CUDA_MANAGED_MEMORY=0.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.