docs: Clarify Node.js specific imports in Next.js register hook migration guide

[lforst]

#11823 rightfully mentioned that the way we recommend people to init the SDK may crash if there are nodejs specific packages being imported in instrumentation.ts.

We could solve this by either changing the way we recommend users to add configuration like so:

import * as Sentry from '@sentry/nextjs';

export async function register() {
  if (process.env.NEXT_RUNTIME === 'nodejs') {
    await import('./sentry.server.config.ts');
  }

  if (process.env.NEXT_RUNTIME === 'edge') {
    await import('./sentry.edge.config.ts');
  }
}

But for now I'd rather keep the base-case simple and add additional explanation for the more complicated case. Maybe we need to switch this up in the future.