code: public <T> AsyncMutateInBuilder arrayAppend(String path, T value) {
          this.mutationSpecs.add(new MutationSpec(Mutation.ARRAY_PUSH_LAST, path, value));
          return this;
      }
docs: Append to an existing array, pushing the value to the back/last position in
  the array.
  
  @param path the path of the array.
  @param value the value to insert at the back of the array.