Update builder management docs to warn that globally setting a builder will force all apps to use that builder

[josegonzalez]

Link to the documentation page

https://dokku.com/docs/deployment/builders/builder-management/

Documentation problem

Due to how builder detection works, a global builder will force-set the builder for all apps. We should warn when thats set (and also potentially output a warning in the detection itself).

dokku report

No response