tab indentation comment (#6079)
## Summary
Updated doc comment for `tab_indentation.rs`. Online docs also benefit
from this update.
## Test Plan
Checked docs via
[mkdocs](https://github.com/astral-sh/ruff/blob/389fe13c934fe73679474006412b1eded4a2cad0/CONTRIBUTING.md?plain=1#L267-L296)
