Clean up within xr_engine profiles and docs

[pawel-wolff]

Description

This PR fixes the following issues:

• Docstring on to_xarray() and open_dataset() concerning attrs corrected and expanded
• The profile-defined xr_engine keyword attrs uses format-agnostic metadata instead of GRIB keys. A related test was improved.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.56%. Comparing base (e015000) to head (c815a37).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1006   +/-   ##
========================================
  Coverage    81.56%   81.56%           
========================================
  Files          236      236           
  Lines        15510    15510           
  Branches       756      755    -1     
========================================
+ Hits         12650    12651    +1     
  Misses        2634     2634           
+ Partials       226      225    -1     

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