DOC: Move API breaking to appropriate sections
As discussed in https://github.com/pandas-dev/pandas/issues/34801, this moves some items from our "API breaking changes" section to either bug fixes or enhancements. I've only moved the ones we can hopefully get consensus on as bug fix / enhancement. But let me know if any of these need further discussion on whether they're an API breaking change.

As requested in https://github.com/pandas-dev/pandas/pull/34865/files/a1130882c28ac17be013287fd5abef40841e1d8e#diff-e763086372257ed7ca944d8bc5042a04 Also cc @jorisvandenbossche for confirmation that this all look like bug fixes. I need to close this for a while.