The Documentation is very vague on the compile part.

It says:
"The service container can be compiled for various reasons."

The issue here is "can be" because it implies that this is not required at all.
If you make use of DefinitionDecoration described here
http://symfony.com/doc/current/components/dependency_injection/parentservices.html

You have to compile the container to make the decoration work.
I filed an Issue for this as well but an update for the documentation would be great.
Just to make this compile thing a little bitte clearer.

Related Issue
#10034

Cheers
Sebastian