dnf5daemon: Document before_begin / after_complete signals

dnf5daemon-server/dbus/interfaces/org.rpm.dnf.v0.rpm.Rpm.xml

@@ -188,6 +188,26 @@ along with libdnf.  If not, see <https://www.gnu.org/licenses/>.
         <arg name="options" type="a{sv}" direction="in" />
     </method>
 
+    <!--
+        transaction_before_begin:
+        @total: number of elements in the rpm transaction
+
+        Send right before the rpm transaction is run.
+    -->
+    <signal name="transaction_before_begin">
+        <arg name="total" type="t" />
+    </signal>
+
+    <!--
+        transaction_after_complete:
+        @success: true if the rpm transaction was completed successfully
+
+        Send right after the rpm transaction run finished.
+    -->
+    <signal name="transaction_after_complete">
+        <arg name="success" type="b" />
+    </signal>
+
     <!--
         transaction_action_start:
         @nevra: full NEVRA of the package