Explain prerelease security upgrade limits (#99230)
Stacked on #99223.
### Why?
Stable security advisories can misclassify prerelease versions because
prerelease ordering does not reliably establish which security fixes
they contain. A blocked upgrade needs a clear way to continue on the
installed channel.
### How?
Explain the advisory limitation in the security error and give the full
`npx next@canary upgrade --ai=latest` command, with instructions to run
it from the app directory. Give unsupported Future Defaults requests the
same latest-channel recovery command.
<!-- NEXT_JS_LLM -->
