[8.x] Add client credentials grant note

[martinbean]

Someone on the Laracasts forums claimed protecting routes with the client credentials grant type was confusing because they had read the “Protecting Routes” section, so had erroneously added auth:api to their routes, instead of using the client middleware as instructed in the “Client Credentials Grant Tokens” section.

This attempts to mitigate that misunderstanding by adding a note highlighting if you are using the client credentials grant type, to use the client middleware and not the auth:api middleware.