[Logger] Add placeholders for dynamic data

[alanpoulain]

From @Crell's post:

Symfony's docs team can include a better example that includes a placeholder on the logging page. It doesn't have to be long, just one more example at the top showing how the placeholder works.

So here it is 😄

[chalasr]

Cool cool, thanks!

[javiereguiluz]

Alan, thanks for this important contribution! And thanks to reviewers too.

Please note that while merging we reworded this a bit (1c33f40). The issue was that the whole explanation about placeholders was too long for a code comment.

So, we split it in two parts:

• Short code comment explaining what are placeholders and how to add them
• A proper text explanation below highlighting the main reasons why it's recommended to use placeholders

Thanks!

[alanpoulain]

No problem, it's even clearer this way, thank you Javier.

[Crell]

Excellent! Thank you everyone. I will update my blog post accordingly. 😄