update contract.encode_abi signature in docs and migration guide

Author: pacrob

docs/migration.rst

@@ -333,7 +333,7 @@ Miscellaneous Changes
   without checking if the ``geth.ipc`` file exists.
 - ``Web3.is_address()`` returns ``True`` for non-checksummed addresses.
 - ``Contract.encodeABI()`` has been renamed to ``Contract.encode_abi()``. The ``fn_name``
-  argument has been changed to ``abi_element_name``.
+  argument has been changed to ``abi_element_identifier``.
 - JSON-RPC responses are now more strictly validated against the JSON-RPC 2.0
   specification while providing more informative error messages for invalid responses.
 

docs/web3.contract.rst

@@ -323,10 +323,10 @@ Each Contract Factory exposes the following methods.
         filter_builder.from_block = "latest"
         filter_builder.from_block = 0  # raises a ValueError
 
-.. py:classmethod:: Contract.encode_abi(fn_name, args=None, kwargs=None, data=None)
+.. py:classmethod:: Contract.encode_abi(abi_element_identifier, args=None, kwargs=None, data=None)
 
     Encodes the arguments using the Ethereum ABI for the contract function that
-    matches the given ``fn_name`` and arguments ``args``. The ``data`` parameter
+    matches the given ``abi_element_identifier`` and arguments ``args``. The ``data`` parameter
     defaults to the function selector.
 
     .. code-block:: python

newsfragments/3473.docs.rst

@@ -0,0 +1 @@
+Update ``contract.encode_abi`` signature in docs and migration guide