doc: document socket protocol change

* doc: document socket protocol change

* spelling

Author: holmanb

doc/rtd/reference/breaking_changes.rst

@@ -11,7 +11,7 @@ releases.
     many operating system vendors patch out breaking changes in
     cloud-init to ensure consistent behavior on their platform.
 
-25.4
+26.1
 ====
 
 Openstack bond names are no longer hard-coded to bond0, bond1, ....
@@ -20,9 +20,19 @@ Instead they are set to whatever value is provided by network_data.json.
 25.3
 ====
 
-**For cloud-init downstream packagers**: the  packaging build backend changed
-to meson and away from python's setuptools.distutils in response to
-`PEP-0632's deprecation of distutils`_.  Meson packaging may not be identical
+Systemd unit file changes
+-------------------------
+
+The socket protocol was changed for better compatibility with alternatives
+to ``openbsd-netcat``, including ``nmap``'s ``ncat -U``. Downstreams that
+provide a custom systemd ``ExecStart=`` statement will need to update.
+
+New build system: meson
+-----------------------
+
+The packaging build backend changed
+from python's setuptools.distutils to meson in response to
+`PEP-0632's deprecation of distutils`_.  Meson packaging will not be identical
 to previous binary packages generated by python's setuptools. Downstream
 packagers should validate the final generated package to ensure
 unexpected package deltas do not exist.