Tweak description templates docs note

What does this MR do?

Edit feature flag notes in description templates doc to follow https://docs.gitlab.com/ee/development/documentation/feature_flags.html#features-that-became-enabled-by-default

Source (internal):

I have a dear customer asking me whether or not it would be safe for them to enable the feature flag inherited_issuable_templates in their 13.10 environment or if they should wait until they upgrade to 13.12? The documentation still reads as if it is not production ready. Is that still the case or should the documentation be updated?

We had the code in since 13.9 behind default: off , but in 13.10 we added a fix. FF was then set it to default: enabled in 13.11 . So either set the FF to enabled in 13.10 or upgrade to 13.11+ should both work.

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