Migrate --pretend can actually create schema unless protected $connection is included in the migration

[phpguru]

In Issue 36596 it was reported that the php artisan migrate --pretend command will create schema elements on non-default database connections if the migration doesn't contain a protected $connection declaration.

This PR seeks to improve the Laravel 10.x documentation to address this apparent bug with php artisan migrate --pretend discussed in that thread.

@taylorotwell actually said, in part,

I'll make sure we are documenting this.

This PR seeks to make good on that promise, and prevent others (like me) from missing this information for any project with multiple databases.

[taylorotwell]

No plans to add this atm.

[phpguru]

@taylorotwell would you accept a PR against master? The lack of documentation around --pretend with multiple databases actually caused some trouble for us, which is how I found Issue 36596. This PR adds two sentences to the docs.