DOC: Add strings for dtypes in basic.rst
- [x] closes #30590
- [ ] tests added / passed
    - N/A
- [ ] passes `black pandas`
    - N/A
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
    - N/A
- [ ] whatsnew entry
    - N/A

Decided to add all valid strings for all the types in the table about dtypes.  I didn't want to decide which ones to leave out, and if we want to leave some of them out, we should decide whether they should be removed from the code as well (e.g., `'Sparse[int, 0]'`)

Had to reformat the table so it would look nice, by splitting the list of strings into multiple lines (e.g., a merged cell)

