add exception info + fix object types in Core (#2865)

* add exception info + fix object types in Core

* Apply suggestions from peer review

Co-Authored-By: Ron Petrusha <[email protected]>

Author: mairaw

xml/System.Security.Cryptography.X509Certificates/X509Certificate2.xml

@@ -473,6 +473,7 @@
         <param name="context">Contextual information about the source of the stream to be deserialized.</param>
         <summary>Initializes a new instance of the <see cref="T:System.Security.Cryptography.X509Certificates.X509Certificate2" /> class using the specified serialization and stream context information.</summary>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.PlatformNotSupportedException">.NET Core only: In all cases.</exception>
         <permission cref="T:System.Security.Permissions.SecurityPermission">for accessing unmanaged code. Security action: <see cref="F:System.Security.Permissions.SecurityAction.InheritanceDemand" />. Associated enumeration: <see cref="F:System.Security.Permissions.SecurityPermissionFlag.UnmanagedCode" /></permission>
       </Docs>
     </Member>
@@ -1795,7 +1796,7 @@
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- Currently this property supports only RSA or DSA keys, so it returns either an <xref:System.Security.Cryptography.RSACryptoServiceProvider> or a <xref:System.Security.Cryptography.DSACryptoServiceProvider> object. If no private key is associated with the certificate, it returns `null`.  
+ Currently this property supports only RSA or DSA keys, so it returns either an <xref:System.Security.Cryptography.RSA> or a <xref:System.Security.Cryptography.DSA> object in .NET Core or an <xref:System.Security.Cryptography.RSACryptoServiceProvider> or a <xref:System.Security.Cryptography.DSACryptoServiceProvider> object in .NET Framework. If no private key is associated with the certificate, it returns `null`.  
   
 > [!WARNING]
 >  If you set this property to `null` or to another key without first deleting it, a private key file is left on the disk.  
@@ -1818,6 +1819,7 @@
         <exception cref="T:System.NotSupportedException">The key algorithm for this private key is not supported.</exception>
         <exception cref="T:System.Security.Cryptography.CryptographicUnexpectedOperationException">The X.509 keys do not match.</exception>
         <exception cref="T:System.ArgumentException">The cryptographic service provider key is <see langword="null" />.</exception>
+        <exception cref="T:System.PlatformNotSupportedException">.NET Core only: An attempt was made to set this property.</exception>
       </Docs>
     </Member>
     <Member MemberName="PublicKey">
@@ -2403,4 +2405,4 @@
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>