Backport PR #3328 on branch 1.10.x (Align `get.obs_df`’s docs with its code)

[meeseeksmachine]

Backport PR #3328: Align get.obs_df’s docs with its code

[codecov[bot]]

Codecov Report

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

Project coverage is 76.94%. Comparing base (7df4ee8) to head (915795f). Report is 1 commits behind head on 1.10.x.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 1.10.x #3334 +/- ## ======================================= Coverage 76.94% 76.94% ======================================= Files 109 109 Lines 12459 12463 +4 ======================================= + Hits 9586 9590 +4 Misses 2873 2873 ``` | [Files with missing lines](https://app.codecov.io/gh/scverse/scanpy/pull/3334?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/\_utils/\_\_init\_\_.py](https://app.codecov.io/gh/scverse/scanpy/pull/3334?src=pr&el=tree&filepath=src%2Fscanpy%2F_utils%2F__init__.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse#diff-c3JjL3NjYW5weS9fdXRpbHMvX19pbml0X18ucHk=) | `75.41% <ø> (ø)` | | | [src/scanpy/get/get.py](https://app.codecov.io/gh/scverse/scanpy/pull/3334?src=pr&el=tree&filepath=src%2Fscanpy%2Fget%2Fget.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse#diff-c3JjL3NjYW5weS9nZXQvZ2V0LnB5) | `92.22% <100.00%> (+0.16%)` | :arrow_up: | | [src/scanpy/tools/\_rank\_genes\_groups.py](https://app.codecov.io/gh/scverse/scanpy/pull/3334?src=pr&el=tree&filepath=src%2Fscanpy%2Ftools%2F_rank_genes_groups.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse#diff-c3JjL3NjYW5weS90b29scy9fcmFua19nZW5lc19ncm91cHMucHk=) | `94.52% <ø> (ø)` | |