Add missing docs for System.Runtime.InteropServices (#8448)

Author: jkoritzinsky

xml/System.Runtime.InteropServices.Marshalling/ContiguousCollectionMarshallerAttribute.xml

@@ -37,7 +37,7 @@
       </AssemblyInfo>
       <Parameters />
       <Docs>
-        <summary>To be added.</summary>
+        <summary>Constructs a default instance of the attribute.</summary>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>

xml/System.Runtime.InteropServices.Marshalling/ReadOnlySpanMarshaller`2+ManagedToUnmanagedIn.xml

@@ -155,7 +155,7 @@ The `buffer` must not be movable - that is, it should not be
       <Parameters />
       <Docs>
         <summary>Returns a reference to the marshalled array.</summary>
-        <returns>To be added.</returns>
+        <returns>A reference that can be pinned and directly passed to unmanaged code.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
@@ -223,7 +223,7 @@ The `buffer` must not be movable - that is, it should not be
       <Parameters />
       <Docs>
         <summary>Returns the unmanaged value representing the array.</summary>
-        <returns>To be added.</returns>
+        <returns>The unmanaged value representing the array.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>

xml/System.Runtime.InteropServices.Marshalling/SpanMarshaller`2+ManagedToUnmanagedIn.xml

@@ -54,7 +54,7 @@
         <ReturnType>System.Int32</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>Gets the size of the caller-allocated buffer to allocate.</summary>
         <value>To be added.</value>
         <remarks>To be added.</remarks>
       </Docs>
@@ -155,7 +155,7 @@ The `buffer` must not be movable - that is, it should not be
       <Parameters />
       <Docs>
         <summary>Returns a reference to the marshalled array.</summary>
-        <returns>To be added.</returns>
+        <returns>A reference that can be pinned and directly passed to unmanaged code.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
@@ -223,7 +223,7 @@ The `buffer` must not be movable - that is, it should not be
       <Parameters />
       <Docs>
         <summary>Returns the unmanaged value representing the array.</summary>
-        <returns>To be added.</returns>
+        <returns>The unmanaged value representing the array.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>

xml/ns-System.Runtime.InteropServices.Marshalling.xml

@@ -1,6 +1,6 @@
 <Namespace Name="System.Runtime.InteropServices.Marshalling">
   <Docs>
-    <summary>To be added.</summary>
+    <summary>Contains support types for source-generated interop, such as <see cref="T:System.Runtime.InteropServices.Marshalling.MarshalUsingAttribute" />.</summary>
     <remarks>To be added.</remarks>
   </Docs>
 </Namespace>