message: Make doc clearer on custom op gradient metadata.
  Summary:
  Note that gradient builders have its own metadata functions.
  Ref T21013
  Test Plan: No functional changes, doc update.
  Reviewers: #tensorflow, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved, davidn
  Maniphest Tasks: T21013
diff: @@ -59,7 +59,8 @@ contain a separate function with the same name as the forward pass builder.
  The function must be given the same name as the forward operation with ``_grad``
  appended. The signature of the builder function is slightly different, as it
  takes the forward pass inputs and outputs as arguments, as well as the
  -gradient outputs.
  +gradient outputs. Gradient builders have their own metadata functions.
  +E.g. metadata function name for the example below will be ``Build_grad_metadata``.
  .. code-block:: cpp
  @@ -124,7 +125,7 @@ The arguments are:
  * ``num_inputs``: indicates how many inputs are on the operation.
  -The function should fill in the values of the first three arguments, which
  +The function should fill in the values of the first four arguments, which
  are all reference types.
  