DOC: move 'Other API changes' under correct section
The "Other API changes" bullet points were split in two (one part under "Other API changes", and one part directly under "Backwards incompatible API changes"), and in previous whatsnew files, the "Other API changes" is also a subsection of "Backwards incompatible API changes".

So moved a few things around to make this consistent.
This was inspired by looking at https://github.com/pandas-dev/pandas/issues/34801, but this PR does not yet "fix" anything, just cleans up the organization.