[OAuth] Qt OAuth2 vendor specific remarks documentation

The major OAuth providers such as Google, Microsoft, Apple, Reddit, and Facebook, each have their slightly differing implementation details.

Qt documentation can't exhaustively document these nuances, and furthermore they change over time.
However, to help developers get started, some remarks would be useful.

In practice this would be a list of things to check and to look out for; URI shemes, loopbacks, nonce, PKCE, etc.