Update StrongNameKeyPair docs (#6656)

Add PlatformNotSupportedException for APIs not supported in .NET Core

Author: MSDN-WhiteKnight

xml/System.Reflection/StrongNameKeyPair.xml

@@ -231,6 +231,7 @@
         <exception cref="T:System.ArgumentNullException">
           <paramref name="keyPairContainer" /> is <see langword="null" />.</exception>
         <exception cref="T:System.Security.SecurityException">The caller does not have the required permission.</exception>
+        <exception cref="T:System.PlatformNotSupportedException">.NET 5+ and .NET Core only: This member is not supported.</exception>
         <altmember cref="T:System.Array" />
       </Docs>
     </Member>
@@ -283,6 +284,7 @@
   
  ]]></format>
         </remarks>
+        <exception cref="T:System.PlatformNotSupportedException">.NET 5+ and .NET Core only: This member is not supported.</exception>
         <related type="Article" href="/dotnet/standard/serialization/xml-and-soap-serialization">XML and SOAP Serialization</related>
       </Docs>
     </Member>
@@ -327,6 +329,7 @@
         <summary>Gets the public part of the public key or public key token of the key pair.</summary>
         <value>An array of type <see langword="byte" /> containing the public key or public key token of the key pair.</value>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.PlatformNotSupportedException">.NET 5+ and .NET Core only: This member is not supported.</exception>
         <altmember cref="T:System.Array" />
         <altmember cref="T:System.String" />
       </Docs>
@@ -371,6 +374,7 @@
         <param name="sender">The object that initiated the callback.</param>
         <summary>Runs when the entire object graph has been deserialized.</summary>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.PlatformNotSupportedException">.NET 5+ and .NET Core only: This member is not supported.</exception>
       </Docs>
     </Member>
     <Member MemberName="System.Runtime.Serialization.ISerializable.GetObjectData">
@@ -423,6 +427,7 @@
         <remarks>To be added.</remarks>
         <exception cref="T:System.ArgumentException">
           <paramref name="info" /> is <see langword="null" />.</exception>
+        <exception cref="T:System.PlatformNotSupportedException">.NET 5+ and .NET Core only: This member is not supported.</exception>
       </Docs>
     </Member>
   </Members>