Add a note about imported parameters not being immediately available

[javiereguiluz]

This issue was first reported in symfony/symfony#3845 and further discussed in symfony/symfony#9361

The current limitation of the container parameters is as follows. If you have the following parameters.yml file:

# app/config/parameters.yml
parameters:
    my_type: foo

Then the following won't work:

# app/config/config.yml
imports:
    - { resource: parameters.yml }
    - { resource: "%my_type%/security.yml" }

Resolved parameters aren't available immediately, so the my_type parameter imported from parameters.yml cannot be used for example as part of the next file imports.

This doesn't work either for kernel parameters:

imports:
    - { resource: "%kernel.root_dir%/parameters.yml" } # <-- this doesn't work as expected

[wouterj]

Fixed by @xabbuh in #4226