DOC: Clarifying brackets vs. parentheses
- [x] closes #56972
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit). (Doc edits only)
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions. (Doc edits only)
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature. (Doc edits only)

In addition to the clarification requested in #56972, I adjusted the language where square brackets were specified elsewhere in the documentation to match "square brackets ``[]``", with the exception of one file.

In the data subsetting tutorial guide (`doc/source/getting_started/intro_tutorials/03_subset_data.rst`),  square brackets are specified towards the beginning of the guide, and "selection brackets" was used elsewhere, often to specify outer vs. inner square brackets (i.e. `df[['ColA', 'ColB']]`)
Thanks @noah-asing!