Add note about enabling the `release_asset_link_editing` feature flag for self-managed users
What does this MR do?
Updates the Release documentation to describe how the release_asset_link_editing
feature flag can be enabled for self-managed users.
Context
The #9427 (closed) feature was completed in %12.10, but the MR to enable the feature didn't quite make it into the release branch (it was merged on the 15th).
This means that self-managed users will not be able to see this feature by default.
More context here: gitlab-com/www-gitlab-com!41001 (comment 326248963)
Related to #9427 (closed)
Edited by Nathan Friend