Document that MR authors should ask production/release teams to explicitly disable any used feature flags before deploy

The following discussion from !7788 (merged) should be addressed:

• @mdelaossa started a discussion: (+5 comments)

  I can imagine some cases where we'd like to default to false (in case we want to, for example, merge the backend portion of a new feature without the frontend). Could we maybe have an optional argument here and in the namespace one?