[Fwb] `framework.php_errors.log` accepts an array mapping E_* constants to log levels

[ogizanagi]

This information is missing from the FrameworkBundle reference: https://symfony.com/doc/current/reference/configuration/framework.html#log and I don't think it's mentioned elsewhere.

Refs:

• [FrameworkBundle] Fixed validation of php_errors.log symfony#39018
• https://github.com/symfony/symfony/blob/c71c8727cc665c9e9b56e299fcfcc0adfbf02bac/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php#L1122
• https://github.com/symfony/symfony/blob/c71c8727cc665c9e9b56e299fcfcc0adfbf02bac/src/Symfony/Component/HttpKernel/EventListener/DebugHandlersListener.php#L49