minor #12318 Add a warning to imports order (tristanbes)

This PR was submitted for the master branch but it was squashed and merged into the 4.3 branch instead (closes #12318).

Discussion
----------

Add a warning to imports order

After spending few hours narrowing this down, I've added a note in the documentation that potentially will save time for developper;

Here's what happened to me: I got a service that didn't worked, it's because the `imports` actually imports the file *before*, so the service definition got overriden with no warning by the Glob rule (`App\:`)

This behaviour is not mentionned on the documentation;

```yaml
// services.yaml

imports:
    - { resource: actions_services.yaml }

services:
    # default configuration for services in *this* file
    _defaults:
        autowire: true
        autoconfigure: true

    App\:
        resource: '../src/*'
        exclude:
          - '../src/{DependencyInjection,Entity,Migrations,Tests,Kernel.php}'
```

```yaml
// actions_services.yaml
services:
    App\Actions\Company\CreateCompany:
        autowire: true
        autoconfigure: false
        parent: 'App\Handler\AbstractHandler'
        calls:
            - [setClient, ['@app\GraphQL\Client']]
            - [setEndpointUrlResolver, ['@app\Resolver\EndpointResolver']]
            - [setSuccessor, ['@app\Actions\User\CreateUser']]
```

<img width="549" alt="tbessoussa_MacBook-Pro-dex-Tristan____workspace_importer-function" src="https://user-images.githubusercontent.com/346010/64860934-eaf0cc80-d62e-11e9-94e9-a0da67d1b366.png">

**Note that the `Calls` section is missing**

Working version:

```yaml

// services.yaml

services:
    # default configuration for services in *this* file
    _defaults:
        autowire: true
        autoconfigure: true

    App\:
        resource: '../src/*'
        exclude:
          - '../src/{DependencyInjection,Entity,Migrations,Tests,Kernel.php}'

    App\Actions\Company\CreateCompany:
        autowire: true
        autoconfigure: false
        parent: 'App\Handler\AbstractHandler'
        calls:
            - [setClient, ['@app\GraphQL\Client']]
            - [setEndpointUrlResolver, ['@app\Resolver\EndpointResolver']]
            - [setSuccessor, ['@app\Actions\User\CreateUser']]
```

<img width="712" alt="tbessoussa_MacBook-Pro-de-Tristan____workspace_importer-function" src="https://user-images.githubusercontent.com/346010/64860840-a9602180-d62e-11e9-9b3a-40e344f18e9f.png">

`Calls` is here, so the service is good to go.

Commits
-------

175b05a Add a warning to imports order

Author: javiereguiluz

service_container/import.rst

@@ -102,6 +102,12 @@ To import this file, use the ``imports`` key from a file that *is* loaded:
 The ``resource`` location, for files, is either a relative path from the
 current file or an absolute path.
 
+.. caution::
+
+   The imported files are loaded first; You should be careful, because it can 
+   lead to some of your services definition to be overridden by rules present
+   inside the file where you do imports
+
 .. include:: /components/dependency_injection/_imports-parameters-note.rst.inc
 
 .. index::