Document TryGetValue from System.Collections.ObjectModel.KeyedCollection (#3431)

* Document TryGetValue from System.Collections.ObjectModel.KeyedCollection

* Update KeyedCollection`2.xml

Author: carlossanlop

xml/System.Collections.ObjectModel/KeyedCollection`2.xml

@@ -1122,11 +1122,12 @@
         <Parameter Name="item" Type="TItem" RefType="out" Index="1" FrameworkAlternate="netcore-2.0;netcore-2.1;netcore-2.2;netcore-3.0;netstandard-2.1" />
       </Parameters>
       <Docs>
-        <param name="key">To be added.</param>
-        <param name="item">To be added.</param>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
+        <param name="key">The key of the item to search in the collection.</param>
+        <param name="item">When this method returns <see langword="true" />, the item from the collection that matches the provided key; when this method returns <see langword="false" />, the <see langword="default" /> value for the type of the collection.</param>
+        <summary>Tries to get an item from the collection using the specified key.</summary>
+        <returns><see langword="true" /> if an item for the specified key was found in the collection; otherwise, <see langword="false" />.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentNullException"><paramref name="key" /> is <see langword="null" />.</exception>
       </Docs>
     </Member>
   </Members>