[Log] Better documentation about log buffering

[sylfabre]

Hello

I googled and successfully found the Symfony documentation about log buffering: https://symfony.com/doc/2.3/cookbook/logging/monolog_email.html

But I experienced the following issues with the doc:

1. The doc is overcomplicated: the page "How to Configure Monolog to Email Errors" is about buffering and sending the log by email. I think that the fingers_crossed handler is out of scope as it adds complexity and is not required to set up Monolog with emails.

2. The doc shows how to use the buffer handler which is very nice. But this handler has more options that are not documented in this page (for instance, flush_on_overflow which defaults to false which a valid but very opinionated choice).

3. I had to search the code to find this list: https://github.com/symfony/monolog-bundle/blob/master/DependencyInjection/Configuration.php#L140 I would expect the Symfony documentation to show at least a link to this PHPDoc documentation as it is quite difficult to find it for newbies!

I don't have the time with my job to fix this and I'm not sure what is the best way to do it, but I wanted to report it to improve the doc.

Anyway, thank you for your amazing work with this awesome framework 💪