Follow-up from "Add filters to blob search with and without Elasticsearch"

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5590#note_80973950

@mdelaossa in the interest of merging this, I moved this discussion to a new issue (it's a docs-only change).

Please add this feature to https://docs.gitlab.com/ee/user/search/advanced_search_syntax.html!