docs: migration-guides: migration-3.4: mention that hardcoded password are supported if hashed

Plaintext/clear passwords are not supported anymore but hashed passwords
still are. Mention that in the migration guide and point to the
appropriate location of documentation.

Cc: Quentin Schulz <foss+yocto@0leil.net>
(From yocto-docs rev: f8b9697ec7bcc188db5ce9e5067bc82c023b79d9)

Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/migration-guides/migration-3.4.rst

@@ -270,6 +270,8 @@ Miscellaneous
   It was being implemented using a custom patch to the ``shadow`` recipe
   which clashed with a ``-P`` option that was added upstream in
   ``shadow`` version 4.9, and in any case is fundamentally insecure.
+  Hardcoded passwords are still supported but they need to be hashed, see
+  examples in :term:`EXTRA_USERS_PARAMS`.
 
 
 .. include:: release-notes-3.4.rst