ReadOnlyMemoryContent class description (#4277)

* ReadOnlyMemoryContent class description

* fix reference

Co-authored-by: Jan Jahoda <jajahoda@.microsoft.com>

Author: Jan Jahoda

xml/System.Net.Http/ReadOnlyMemoryContent.xml

@@ -23,7 +23,7 @@
   </Base>
   <Interfaces />
   <Docs>
-    <summary>To be added.</summary>
+    <summary>Provides HTTP content based on a <see cref="T:System.ReadOnlyMemory&lt;System.Byte&gt;" />.</summary>
     <remarks>To be added.</remarks>
   </Docs>
   <Members>
@@ -49,8 +49,8 @@
         <Parameter Name="content" Type="System.ReadOnlyMemory&lt;System.Byte&gt;" />
       </Parameters>
       <Docs>
-        <param name="content">To be added.</param>
-        <summary>To be added.</summary>
+        <param name="content">The content used to initialize the <see cref="T:System.Net.Http.ReadOnlyMemoryContent" />.</param>
+        <summary>Initializes a new instance of the <see cref="T:System.Net.Http.ReadOnlyMemoryContent" /> class.</summary>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>