Explain NotNull on an input parameter (#3782)

Relates to dotnet/runtime#1891

Author: jcouv

xml/System.Diagnostics.CodeAnalysis/NotNullAttribute.xml

@@ -24,7 +24,7 @@
     </Attribute>
   </Attributes>
   <Docs>
-    <summary>Specifies that an output is not <see langword="null" /> even if the corresponding type allows it.</summary>
+    <summary>Specifies that an output is not <see langword="null" /> even if the corresponding type allows it. Specifies that an input argument was not <see langword="null" /> when the call returns.</summary>
     <remarks>To be added.</remarks>
   </Docs>
   <Members>