Merge branch 'omniauth-doc-fix' into 'master'

Add missing "omniauth" prefix to option in docs

This MR is specifically a fix for #3319. I wanted to make my first contribution to GitLab, so I picked a simple one!

This changes the omniauth documentation for the `block_auto_created_users` option to `omniauth_block_auto_created_users`.

Thanks

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closes #3319 

See merge request !1734