Add example of cols=<list> to ?setindexv (#6678)

* fixed#6665

* simplify

---------

Co-authored-by: Michael Chirico <[email protected]>

Author: venom1204

man/setkey.Rd

@@ -142,6 +142,11 @@ setindex(DT, B)
 indices(DT)           # get indices single vector
 indices(DT, vectors = TRUE) # get indices list
 
+# Setting multiple indices at once
+DT = data.table(A = 5:1, B = letters[5:1], C = 10:6)
+setindexv(DT, list(c("A", "B"), c("B", "C")))
+print(DT, show.indices=TRUE)
+
 # Use the dot .(subset_value) syntax with integer keys:
 DT = data.table(id = 2:1)
 setkey(DT, id)