Improve composite docs and changelog.

PiperOrigin-RevId: 730394496
Change-Id: I522cddfc14cc9ad4c06a6efea39eb5d77d0ed9ac

Author: quagla

dm_control/mjcf/schema.xml

@@ -1215,7 +1215,6 @@
             <attribute name="type" type="keyword" valid_values="particle grid cable rope loop cloth" required="true"/>
             <attribute name="count" type="array" array_type="int" array_size="3"/>
             <attribute name="offset" type="array" array_type="float" array_size="3"/>
-            <attribute name="flatinertia" type="float"/>
             <attribute name="vertex" type="string"/>
             <attribute name="initial" type="string"/>
             <attribute name="curve" type="string"/>
@@ -1582,7 +1581,6 @@
                 <attribute name="type" type="keyword" valid_values="particle grid cable rope loop cloth" required="true"/>
                 <attribute name="count" type="array" array_type="int" array_size="3"/>
                 <attribute name="offset" type="array" array_type="float" array_size="3"/>
-                <attribute name="flatinertia" type="float"/>
                 <attribute name="vertex" type="string"/>
                 <attribute name="initial" type="string"/>
                 <attribute name="curve" type="string"/>