Improve UID_IN with variable (done)

[Anurag]

@dmai

At the very least, it should support the uid() function so you can say uid_in(director.film, uid(varName)) .

This is now supported after this PR: https://github.com/dgraph-io/dgraph/pull/5320 albeit with a uid variable. @MichelDiz can you close this and accept the solution. Also probably edit the heading.