Update %env()% documentation

[ro0NL]

See symfony/symfony#23901 + symfony/symfony#23888

Just a ticket for reference now :)

[javiereguiluz]

See also https://symfony.com/blog/new-in-symfony-3-4-advanced-environment-variables

[ro0NL]

It's getting ready :) i think for most users things will work out-of-the-box / as expected.

Nevertheless if you decide to document more advanced stuff when using env vars, i think these points are worth mentioning related to the Config tier;

• no real env lookup, config is validated with dummy env values based on the prefix (string by default)
  • this happens at normalization, merging and finalizing level (main Processor API)
• env placeholder can be detected with normalization closures upfront, and thus can be normalized
  • if the user normalizes a different value; then either the type changed or the string value changed (the placeholder is concatenated or so). In that case processing continues as usual (it doesnt use dummy value as it's a regular string)
• env vars are not compatible with enum nodes because we expect a known value (envs can be anything)
• env vars are not compatible with array nodes (either being prototyped yes/no) because that's part of the semantic structure, thus needs processing (normalizing, merging, validating)

[javiereguiluz]

Fixed by #9507.