[Translation] Update translation.rst

[noniagriconomie]

Related to https://github.com/symfony/symfony/blob/5.x/src/Symfony/Contracts/Translation/TranslatorInterface.php#L24
I propose to add this here, as not all projects uses ICU translations, so documenting the special %count% could be useful

[javiereguiluz]

I'm not against adding this ... but the proposed location feels a bit odd. It's like we abruptly add some content in that section 🤔

[noniagriconomie]

@javiereguiluz yes indeed, what do you think to move this tip after the:

Another complication is when you have translations that may or may not be plural, based on some variable: code example?
as it is question of the word plural here

[javiereguiluz]

Thanks Antoine! At the end I did what you suggested and I moved this to the section that briefly mentions the pluralization of messages.