documentation: Add since tag to xmldocs DTD

Adds the since tag to the documentation DTD so
that individual applications, functions, etc.
can now specify when they were added to Asterisk.

This tag is added at the individual application,
function, etc. level as opposed to at the module
level because modules can expand over time as new
functionality is added, and granularity only
to the module level would generally not be useful.

This enables the ability to more easily determine
when new functionality was added to Asterisk, down
to minor version as opposed to just by major version.
This makes it easier for users to write more portable
dialplan if desired to not use functionality that may
not be widely available yet.

ASTERISK-29896 #close

Change-Id: Ibbb35c702d8038bdc3fd0a944fbfa69384cc15d5

apps/app_reload.c

@@ -39,6 +39,11 @@
 
 /*** DOCUMENTATION
 	<application name="Reload" language="en_US">
+		<since>
+			<version>16.20.0</version>
+			<version>18.6.0</version>
+			<version>19.0.0</version>
+		</since>
 		<synopsis>
 			Reloads an Asterisk module, blocking the channel until the reload has completed.
 		</synopsis>

doc/appdocsxml.dtd

@@ -33,22 +33,22 @@
 
   <!ELEMENT removed_in (#PCDATA)>
 
-  <!ELEMENT application (synopsis?,syntax?,description?,see-also?)>
+  <!ELEMENT application (since?,synopsis?,syntax?,description?,see-also?)>
   <!ATTLIST application name CDATA #REQUIRED>
   <!ATTLIST application language CDATA #REQUIRED>
   <!ATTLIST application module CDATA #IMPLIED>
 
-  <!ELEMENT function (synopsis?,syntax?,description?,see-also?)>
+  <!ELEMENT function (since?,synopsis?,syntax?,description?,see-also?)>
   <!ATTLIST function name CDATA #REQUIRED>
   <!ATTLIST function language CDATA #REQUIRED>
   <!ATTLIST function module CDATA #IMPLIED>
 
-  <!ELEMENT agi (synopsis?,syntax?,description?,see-also?)>
+  <!ELEMENT agi (since?,synopsis?,syntax?,description?,see-also?)>
   <!ATTLIST agi name CDATA #REQUIRED>
   <!ATTLIST agi language CDATA #REQUIRED>
   <!ATTLIST agi module CDATA #IMPLIED>
 
-  <!ELEMENT manager (synopsis?,syntax?,description?,see-also?,responses?)>
+  <!ELEMENT manager (since?,synopsis?,syntax?,description?,see-also?,responses?)>
   <!ATTLIST manager name CDATA #REQUIRED>
   <!ATTLIST manager language CDATA #REQUIRED>
   <!ATTLIST manager module CDATA #IMPLIED>
@@ -62,7 +62,7 @@
   <!ATTLIST managerEvent language CDATA #REQUIRED>
   <!ATTLIST managerEvent module CDATA #IMPLIED>
 
-  <!ELEMENT managerEventInstance (synopsis?,syntax?,description?,see-also?)*>
+  <!ELEMENT managerEventInstance (since?,synopsis?,syntax?,description?,see-also?)*>
   <!ATTLIST managerEventInstance class CDATA #REQUIRED>
 
   <!ELEMENT configInfo (synopsis?,description?,configFile+)>
@@ -102,6 +102,10 @@
   <!ATTLIST ref type (application|function|astcli|link|manpage|filename|agi|manager|managerEvent|configOption) #REQUIRED>
   <!ATTLIST ref module CDATA #IMPLIED>
 
+  <!ELEMENT since (version+)>
+
+  <!ELEMENT version (#PCDATA)>
+
   <!ELEMENT synopsis (#PCDATA)>
 
   <!ELEMENT syntax (parameter|dataType|category|matchInfo|xi:include|channel_snapshot|bridge_snapshot)*>