Document how to annotate member as key in XMl types (#746) (#755)

Signed-off-by: eduponz <eduardoponz@eprosima.com>
(cherry picked from commit f95e5c9)

Co-authored-by: Eduardo Ponz Segrelles <eduardoponz@eprosima.com>

Author: mergify[bot]

code/XMLTester.xml

@@ -4250,6 +4250,9 @@
 </struct>
 <!--><-->
 <struct name="MembersExample">
+<!-->XML-MEMBER_WITH_KEY<-->
+<member name="my_long" type="int32" key="true"/>
+<!--><-->
 <!-->XML-BOUNDEDSTRINGS<-->
 <member name="my_large_string" type="string" stringMaxLength="41925"/>
 <member name="my_large_wstring" type="wstring" stringMaxLength="20925"/>

docs/fastdds/xml_configuration/dynamic_types.rst

@@ -60,6 +60,12 @@ Member types
 Member types are defined as any type that can belong to a `Struct`_ or a `Union`_, or be aliased by a
 `Typedef`_.
 These can be defined by the ``<member>`` XML tag.
+A member can be annotated as ``key`` (equivalent of the IDL's ``@key``) by setting the ``key`` attribute to ``"true"``.
+
+.. literalinclude:: /../code/XMLTester.xml
+  :language: xml
+  :start-after: <!-->XML-MEMBER_WITH_KEY<-->
+  :end-before: <!--><-->
 
 Primitive types
 ***************