[DI] Add select specific key from an array resolved env var documentation

[bobvandevijver]

In symfony/symfony#27157, the key processor is added. This should also be documented, but currently there is no documentation yet (see #8382, with #9507 as work-in-progress MR).

When the initial documentation is merged, the new key processor should also be added

[xabbuh]

Did you mean symfony/symfony#27157?

[bobvandevijver]

I did indeed, good catch!

[nicolas-grekas]

@bobvandevijver would you mind sending a PR now that this is merged?

[bobvandevijver]

@nicolas-grekas I wouldn't, I've put it on my list to do this soon!