Improve DeviceBuffer docs.

[bdice]

Description

I noticed a few errors in the DeviceBuffer docs while using Python's help(rmm.DeviceBuffer). These errors probably weren't noticed because the docs for DeviceBuffer are not publicly exposed. Reviewers: Should we check to ensure that we are exposing docs for this class and other APIs in the rmm.* Python namespace, and add those (in a separate PR)?

Checklist

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

[bdice]

@gpucibot merge