Update SecurityIdentifier.xml (#2231)

Author: slozier

xml/System.Security.Principal/SecurityIdentifier.xml

@@ -236,8 +236,8 @@
         <ReturnType>System.Security.Principal.SecurityIdentifier</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Returns the account domain security identifier (SID) portion from the SID represented by the <see cref="T:System.Security.Principal.SecurityIdentifier" /> object if the SID represents a Windows account SID. If the SID does not represent a Windows account SID, this property returns <see cref="T:System.ArgumentNullException" />.</summary>
-        <value>The account domain SID portion from the SID represented by the <see cref="T:System.Security.Principal.SecurityIdentifier" /> object if the SID represents a Windows account SID; otherwise, it returns <see cref="T:System.ArgumentNullException" />.</value>
+        <summary>Returns the account domain security identifier (SID) portion from the SID represented by the <see cref="T:System.Security.Principal.SecurityIdentifier" /> object if the SID represents a Windows account SID. If the SID does not represent a Windows account SID, this property returns <see langword="null" />.</summary>
+        <value>The account domain SID portion from the SID represented by the <see cref="T:System.Security.Principal.SecurityIdentifier" /> object if the SID represents a Windows account SID; otherwise, it returns <see langword="null" />.</value>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
@@ -851,4 +851,4 @@
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>