Add cryptography PlatformNotSupportedExceptions (#7939)

Author: gewarren

xml/System.Security.Cryptography/ECDiffieHellmanCng.xml

@@ -998,6 +998,7 @@ This instance represents only a public key.</exception>
         <exception cref="T:System.ArgumentOutOfRangeException">
           <paramref name="format" /> specifies an invalid format. The only accepted value is <see cref="F:System.Security.Cryptography.ECKeyXmlFormat.Rfc4050" />.</exception>
         <exception cref="T:System.Security.Cryptography.CryptographicException">All other errors.</exception>
+        <exception cref="T:System.PlatformNotSupportedException">.NET Core and .NET 5+: In all cases.</exception>
       </Docs>
     </Member>
     <Member MemberName="GenerateKey">
@@ -1796,6 +1797,7 @@ Because key sizes do not uniquely identify elliptic curves, the use of the prope
         <remarks>To be added.</remarks>
         <exception cref="T:System.ArgumentOutOfRangeException">
           <paramref name="format" /> specifies an invalid format. The only accepted value is <see cref="F:System.Security.Cryptography.ECKeyXmlFormat.Rfc4050" />.</exception>
+        <exception cref="T:System.PlatformNotSupportedException">.NET Core and .NET 5+: In all cases.</exception>
       </Docs>
     </Member>
     <Member MemberName="TryExportEncryptedPkcs8PrivateKey">

xml/System.Security.Cryptography/ECDiffieHellmanCngPublicKey.xml

@@ -328,6 +328,7 @@
         </remarks>
         <exception cref="T:System.ArgumentNullException">The <paramref name="xml" /> parameter is <see langword="null" />.</exception>
         <exception cref="T:System.ArgumentException">The <paramref name="xml" /> parameter does not specify an <see cref="T:System.Security.Cryptography.ECDiffieHellman" /> key.</exception>
+        <exception cref="T:System.PlatformNotSupportedException">.NET Core and .NET 5+: In all cases.</exception>
       </Docs>
     </Member>
     <Member MemberName="Import">
@@ -407,6 +408,7 @@
   
  ]]></format>
         </remarks>
+        <exception cref="T:System.PlatformNotSupportedException">.NET Core and .NET 5+: In all cases.</exception>
       </Docs>
     </Member>
   </Members>

xml/System.Security.Cryptography/ECDiffieHellmanPublicKey.xml

@@ -417,6 +417,7 @@
         <summary>Serializes the <see cref="T:System.Security.Cryptography.ECDiffieHellmanPublicKey" /> key BLOB to a byte array.</summary>
         <returns>A byte array that contains the serialized Elliptic Curve Diffie-Hellman (ECDH) public key.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.PlatformNotSupportedException">Linux and macOS: In all cases.</exception>
       </Docs>
     </Member>
     <Member MemberName="ToXmlString">
@@ -462,6 +463,7 @@
         <summary>Serializes the <see cref="T:System.Security.Cryptography.ECDiffieHellmanPublicKey" /> public key to an XML string.</summary>
         <returns>An XML string that contains the serialized Elliptic Curve Diffie-Hellman (ECDH) public key.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.PlatformNotSupportedException">.NET Core and .NET 5+: In all cases.</exception>
       </Docs>
     </Member>
     <Member MemberName="TryExportSubjectPublicKeyInfo">