Merge branch 'jramsay-3953-fast-ssh-introduced-docs' into 'master'

Add release added and release backported to CE

See merge request gitlab-org/gitlab-ee!4147

doc/administration/operations/fast_ssh_key_lookup.md

 # Fast lookup of authorized SSH keys in the database
 
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/issues/1631) in 
+> [GitLab Enterprise Edition Standard](https://about.gitlab.com/gitlab-ee) 9.3.
+>
+> [Available in](https://gitlab.com/gitlab-org/gitlab-ee/issues/3953) GitLab
+> Community Edition 10.4.
+
 Regular SSH operations become slow as the number of users grows because OpenSSH
 searches for a key to authorize a user via a linear search. In the worst case,
 such as when the user is not authorized to access GitLab, OpenSSH will scan the