docs/migrating.adoc: document the tarball suffix update for Cargo-fetch packages

We changed the suffix of tarballs for Cargo-fetched packages in
commit [1], which affects out-of-tree packages, so it makes sense
to document that in the migration guide.

[1] e8c8bd9bc5

Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
[Julien: add link to the commit changing the archive format]
Signed-off-by: Julien Olivain <ju.o@free.fr>

docs/manual/migrating.adoc

@@ -258,3 +258,9 @@ from +true+ to +false+, in line with upstream defaults. If you need
 +EnableNetworkConfiguration=true+ to let +iwd+ handle network
 configuration for active wifi connections, adjust +/etc/iwd/main.conf+
 in your build (see xref:rootfs-custom[] for options).
+
+Due to the update of Rust to a version greater than 1.96.1 $comment was
+added to .cargo-checksum.json to clarify that this file is not a
+security mechanism. Tarballs generated by Cargo-fetched packages have
+changed, therefore the suffix of such tarballs has been changed from
++-cargo5+ to +-cargo6+.