Clarification of the neccessary autoloading of plugins...

...in the error message

Added links to the relevant documentation
I don't think I'm making myself clear.

This new error message is inside this:

    if (!empty($plugin) && !Plugin::loaded($plugin)):

So e.g. if you create/move the plugin manually and load the plugin - you won't get any "btw - you derped your composer by doing things manually" hints.