Removes the suggestion to just use the strongly-discouraged-by-Apple `NSAllowsArbitraryLoads`, and instead links to Apple's docs about the various options.

This removes the example entirely since there's no one-size-fits-all case; some clients would just need `NSExceptionDomains` (this seems likely to be the more common case?), others `NSAllowsArbitraryLoadsForMedia`, others `NSAllowsArbitraryLoadsForMedia`+` NSAllowsArbitraryLoads` (to support iOS 9). Given that the narrowest option that works for someone is the best option, having them actually research the options is the best thing to steer people toward, even if that's not as simple as "copy and paste the following".

Also opportunistically removes a couple of obsolete items:
- An incorrect note that `android.permission.INTERNET` is likely already present in the
  main Android manifest file; it was made debug-only a while ago.
- An old note about web version compatibility; we only support Flutter 2+ now anyway.