DOC: Reference level name as Term of HDFStore.select query (#28791)
Add documentation for default level name with HDFStore MultiIndex.

- [x] closes #28791
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
are there any other issues about this (can you do a search) > are there any other issues about this (can you do a search)

I did not find any other issue specifically about this (I did not even find it on StackOverflow). I guess everybody name its index…

I did find the issue #6208 that speficy that you cant use "index" as an level name (still true on master). I guess it is worth a note in section Storing MultiIndex DataFrames. thanks @nrebena 