clarify language around how FetchContent is used in RAPIDS.cmake

[jameslamb]

Description

Reading through, https://docs.rapids.ai/api/rapids-cmake/stable/basics/, I found this sentence a bit confusing:

The rapids-cmake module is designed to be acquired via CMake’s Fetch Content into your project.

Because the code block immediately below it does not actually call FetchContent() and friends.

This PR proposes a change to that language which I think might make it clearer that RAPIDS.cmake is itself using FetchContent().

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [ ] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.
• [x] The cmake-format.json is up to date with these changes.
• [ ] I have added new files under rapids-cmake/
  • [ ] I have added include guards (include_guard(GLOBAL))
  • [ ] I have added the associated docs/ rst file and update the api.rst

[robertmaynard]

/merge