SignedCMS.ComputeSignature change for .NET Core (dotnet#3866)

* Fixes dotnet#3456 * Add remark and update parameter description * Update xml/System.Security.Cryptography.Pkcs/SignedCms.xml Co-Authored-By: Maira Wenzel <mairaw@microsoft.com>

Author: gewarren

xml/System.Security.Cryptography.Pkcs/SignedCms.xml

@@ -655,7 +655,7 @@ The following permissions are required to access the signature key:
  </Parameters>
  <Docs>
  <param name="signer">A <see cref="T:System.Security.Cryptography.Pkcs.CmsSigner" /> object that represents the signer.</param>
- <summary>The <see cref="M:System.Security.Cryptography.Pkcs.SignedCms.ComputeSignature(System.Security.Cryptography.Pkcs.CmsSigner)" /> method creates a signature using the specified signer and adds the signature to the CMS/PKCS #7 message.</summary>
+ <summary>Creates a signature using the specified signer and adds the signature to the CMS/PKCS #7 message.</summary>
  <remarks>
  <format type="text/markdown"><![CDATA[ 
  
@@ -720,12 +720,14 @@ The following permissions are required to access the signature key:
  </Parameters>
  <Docs>
  <param name="signer">A <see cref="T:System.Security.Cryptography.Pkcs.CmsSigner" /> object that represents the signer.</param>
- <param name="silent">This parameter is not used. </param>
+ <param name="silent">.NET Core only: <see langword="true" /> to request opening keys with PIN prompts disabled, where supported; otherwise, <see langword="false" />. In .NET Framework, this parameter is not used and a PIN prompt is always shown, if required.</param>
  <summary>Creates a signature using the specified signer and adds the signature to the CMS/PKCS #7 message. </summary>
  <remarks>
  <format type="text/markdown"><![CDATA[ 
  
-## Remarks 
+## Remarks
+
+.NET Core only: The `silent` parameter has no effect if the <xref:System.Security.Cryptography.Pkcs.CmsSigner.PrivateKey?displayProperty=nameWithType> value is not null. The provided value controls whether or not computing the signature shows a PIN prompt. Even when the <xref:System.Security.Cryptography.Pkcs.CmsSigner.PrivateKey> property is null and the `silent` parameter is set to `true`, some combinations of operating system and signer options can still result in a PIN prompt.
  
 ## .NET Framework Security 
  The following permissions are required to display the user interface: