Consider updating Feature Flag documentation around: `Add parenthesis support for if: conditions`

The following discussions from !37574 (merged) should be addressed:

@marcel.amirault started a discussion:
From https://docs.gitlab.com/ee/development/documentation/feature_flags.html we would normally add this in it's own section. That said, if you think it'll get enabled by default soon, or the flag removed quickly, this is good as-is, I think.

If the feature flag will stick around for at least one release, then:
```
##### Enable or disable parenthesis support for variables **(CORE ONLY)**

The feature is currently deployed behind a feature flag that is **disabled by default**.
```

Like the other message, if the feature flag is going to stick around a while, we should note it. If not, then ignore this suggestion:

> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/230938) in GitLab 13.3
> - It's deployed behind a feature flag, disabled by default.
> - It's disabled on GitLab.com.
> - It's not recommended for production use.
> - To use it in GitLab self-managed instances, ask a GitLab administrator to [enable it](enable-or-disable-parenthesis-support-for-variables-core-only). **(CORE ONLY)**

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Edited Mar 13, 2023 by 🤖 GitLab Bot 🤖