CLJS-3143: assoc docstring regarding growing vector

Author: LeaveNhA

src/main/cljs/cljs/core.cljs

@@ -1971,7 +1971,7 @@ reduces them without incurring seq initialization"
   "assoc[iate]. When applied to a map, returns a new map of the
    same (hashed/sorted) type, that contains the mapping of key(s) to
    val(s). When applied to a vector, returns a new vector that
-   contains val at index."
+   contains val at index. Note - index must be <= (count vector)."
   ([coll k v]
     (if-not (nil? coll)
       (-assoc coll k v)