Updated Harmony Integrate Docs to better match interface to Harmonypy package

[DaminK]

Fixed harmony documentation where a crucial feature of Harmony, namely the integration w.r.t. multiple covariates was not mentioned despite being supported by the interface. Harmony can take strings and lists of strings as its "vars_use" parameter which is named "key" in the Scanpy interface.

• [ ] Closes #
• [X] Tests included or not required because: Small non-breaking change to docs
• [X Release notes not necessary because: Change docs to better reflect current code behaviour/features

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 76.58%. Comparing base (02cb8c0) to head (f7e0ec4).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3361 +/- ## ======================================= Coverage 76.58% 76.58% ======================================= Files 111 111 Lines 12862 12862 ======================================= Hits 9850 9850 Misses 3012 3012 ``` | [Files with missing lines](https://app.codecov.io/gh/scverse/scanpy/pull/3361?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse) | Coverage Δ | | |---|---|---| | [src/scanpy/external/pp/\_harmony\_integrate.py](https://app.codecov.io/gh/scverse/scanpy/pull/3361?src=pr&el=tree&filepath=src%2Fscanpy%2Fexternal%2Fpp%2F_harmony_integrate.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse#diff-c3JjL3NjYW5weS9leHRlcm5hbC9wcC9faGFybW9ueV9pbnRlZ3JhdGUucHk=) | `86.66% <ø> (ø)` | |