Document the cache.prefix_seed option

[xabbuh]

When you deploy multiple applications into the same path (on different servers) that share the same cache backend you need to configure the cache.prefix_seed option to prevent naming collisions.

[xabbuh]

@nicolas-grekas Anything from your side to add here?

[nicolas-grekas]

You also need to set this option when using blue/green deployment strategies.

[nicolas-grekas]

And more generally, when you need to completely abstract out the actual deployment directory (think offline cache warmups).

[nicolas-grekas]

See symfony/symfony#20616