message: Update docs for API
  Summary:
  Make the sections per release.
  ref T16509
  Test Plan: CI
  Reviewers: babakk, jamiep, #tensorflow, simonl, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved
  Maniphest Tasks: T16509
diff: API changes
  -----------
  +Release 2.1
  +~~~~~~~~~~~
  +
  +The following changes have been made to the TensorFlow API in the Poplar SDK version 2.1.
  +This may require you to change your code.
  +
  +Breaking changes
  +________________
  +
  +.. warning::
  +
  + These will require changes to any code that uses them.
  +
  +We have removed several items that have been deprecated for at least one
  +release.
  +
  +Non-breaking changes
  +____________________
  +
  +These changes are recommended.
  +
  +
  +Release 2.0
  +~~~~~~~~~~~
  +
  The following changes have been made to the TensorFlow API in the Poplar SDK version 2.0.
  This may require you to change your code.
  Breaking changes
  -~~~~~~~~~~~~~~~~
  +________________
  .. warning::
  @@ -94,7 +119,7 @@ The following deprecated namespace has been removed:
  Non-breaking changes
  -~~~~~~~~~~~~~~~~~~~~
  +____________________
  These changes are recommended.
  