Update docs for projects_not_indexed rake task

What does this MR do and why?

From slack thread (internal) and customer ticket (internal)

It was reported that gitlab:elastic:index_projects_status and gitlab:elastic:projects_not_indexed + the Advanced Search UI area claim that all projects are indexed, however... the actual indices for projects is completely empty

This MR updates the docs for one of the rake tasks to make it clear that the task is checking repository data only

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