api: add 'Since' tags

These are not actively used by gdbus-codegen or gtk-doc, but they're
helpful anyway so that users know when a given API method was
introduced.

introspection/org.freedesktop.ModemManager1.xml

@@ -24,6 +24,8 @@
         ScanDevices:
 
         Start a new scan for connected modem devices.
+
+        Since: 1.0
     -->
     <method name="ScanDevices" />
 
@@ -32,6 +34,8 @@
         @level: One of <literal>"ERR"</literal>, <literal>"WARN"</literal>, <literal>"INFO"</literal>, <literal>"DEBUG"</literal>.
 
         Set logging verbosity.
+
+        Since: 1.0
     -->
     <method name="SetLogging">
       <arg name="level" type="s" direction="in" />
@@ -101,6 +105,8 @@
             </listitem>
           </varlistentry>
         </variablelist>
+
+        Since: 1.8
     -->
     <method name="ReportKernelEvent">
       <arg name="properties" type="a{sv}" direction="in" />
@@ -121,6 +127,8 @@
         This operation binds the inhibition request to the existence of the
         caller in the DBus bus. If the caller disappears from the bus, the
         inhibition will automatically removed.
+
+        Since: 1.10
     -->
     <method name="InhibitDevice">
       <arg name="uid"     type="s" direction="in" />
@@ -131,6 +139,8 @@
         Version:
 
         The runtime version of the ModemManager daemon.
+
+        Since: 1.10
     -->
     <property name="Version" type="s" access="read" />