Metaplex docs have a great explanation and diagrams for understanding the relationship between wallets, token accounts, and mints. Recommend looking through this page: https://docs.metaplex.com/programs/understanding-programs
Token accounts have an owner field, the docs you referenced seem to be saying that there will be an option to make the owner field immutable, and ATAs will have the owner immutable by default
Since the address for an Associated Token Account is derived using a token mint and a user's publickey, you would not want the owner of the Associated Token Account to be anything other than the user's publickey.
However, if a token account that was initialized with a randomly generated keypair, you may want to have the ability to change the owner the token account.
