Better explanation of `append` and `overwrite` in `to_parquet`

As mentioned in https://github.com/dask/dask-expr/pull/1051#discussion_r1589112791, I have trouble understanding what the two options do and how they are _not_ the inverse of one another. I suspect users face the same problem.

Start by reading the existing `to_parquet` documentation and the linked discussion in pull request #1051. Clarify what `append` and `overwrite` each do and how they differ, then ensure the documentation explains their relationship clearly.

Written by the indexing model from the issue text.