Add explicit section about type overriding for v15 (#72866)
This breaks out the type overriding note from a small good to know
section to it's own section as it's fairly important and easy to miss
currently. Also the minimal note isn't getting picked up in our search
currently.
x-ref: [slack
thread](https://vercel.slack.com/archives/C0676QZBWKS/p1731602395153929)
