Document host parameter and external parameters in "How to Use PdoSessionHandler..."

[pueppiblue]

I recently had the use case to store sessions via pdo on a docker grid.
I noticed the doctrine/pdo_session_storage.rst documentation is missing the option
to configure a host on the PdoSessionHandler and that
the service configuration does not use external environment parameters.

• add host parameter to first argument
  useful when database is on another host e.g. docker grid
• wrap parameter values in %env()%
• add note how to use external parameters

[DylanDelobel]

The trailing comma in the last element of the arrays is missing, look #8325

[pueppiblue]

$container->autowire(PdoSessionHandler::class) 
            ->setArguments( \
                array( \
                    'mysql:dbname=%env(MYDATABASE_NAME)%, host=%env(MYDATABASE_HOST)%',
                    array(
                        'db_table' => '%env(SESSIONS_TABLE)%',
                        'db_username' => '%env(MYDATABASE_USERNAME)%',
                        'db_password' => '%env(MYDATABASE_PASSWORD)',
                    ), // awkward comma
                )
            );

To be really consistent, I'd to add another comma, but for me it looks kind of awkward after the closing brace.
What would you think?

[javiereguiluz]

@pueppiblue thanks a lot for this contribution. I totally agree with the addition of the missing host option. Thanks for that!

However, I've reverted the other changes mainly for two reasons:

• To not duplicate the explanation about env vars of the /configuration/external_parameters.rst article that you linked here.
• Because it's not that simple to do now that Symfony promotes the use of database DSNs in the DATABASE_URL env var. We no longer have individual env vars for database name, host, use, etc.

In any case, thanks for your contribution and we've merged it in 3.4 (later we'll merge it in the other branches). Congrats on your Symfony Docs contribution!