DOC: add missing functions (#529)

Author: lucascolley

docs/api-reference.md

@@ -7,6 +7,7 @@
     :toctree: generated
 
     apply_where
+    argpartition
     at
     atleast_nd
     broadcast_shapes
@@ -15,11 +16,13 @@
     default_dtype
     expand_dims
     isclose
+    isin
     kron
     nan_to_num
     nunique
     one_hot
     pad
+    partition
     setdiff1d
     sinc
 ```