Merge pull request dotnet#2085 from GuardRex/guardrex/RSACng-RSAOpenSsl-updates

Add notes to use platform agnostic RSA.Create

Author: BillWagner

xml/System.Security.Cryptography/RSACng.xml

@@ -33,6 +33,9 @@
 -   The key used by <xref:System.Security.Cryptography.RSACng> is managed by a separate <xref:System.Security.Cryptography.CngKey> object. In contrast, <xref:System.Security.Cryptography.RSACryptoServiceProvider> has a key that is directly tied to the operations of the type itself.  
   
 -   <xref:System.Security.Cryptography.RSACng> performs such operations as signing and verifying signatures by using properties of the <xref:System.Security.Cryptography.RSACng> object, just as <xref:System.Security.Cryptography.ECDsaCng> uses its object properties to control signing and verification operations.  
+
+> [!NOTE]
+> The <xref:System.Security.Cryptography.RSACng> class is an implementation of the RSA algorithm using the Windows CNG libraries and isn't available on operating systems other than Windows. For applications which aren't doing Windows interop, you're encouraged to use <xref:System.Security.Cryptography.RSA.Create%2A?displayProperty=fullName> instead of referencing this type directly.
   
  ]]></format>
     </remarks>

xml/System.Security.Cryptography/RSAOpenSsl.xml

@@ -13,7 +13,15 @@
   <Interfaces />
   <Docs>
     <summary>To be added.</summary>
-    <remarks>To be added.</remarks>
+    <remarks>
+      <format type="text/markdown"><![CDATA[  
+  
+## Remarks  
+> [!NOTE]
+> The <xref:System.Security.Cryptography.RSAOpenSsl> class is an implementation of the RSA algorithm using OpenSSL. It isn't available on Windows and is only available on other operating systems when OpenSSL is installed. For applications which aren't doing OpenSSL-specific interop, you're encouraged to use <xref:System.Security.Cryptography.RSA.Create%2A?displayProperty=fullName> instead of referencing this type directly.
+  
+ ]]></format>
+    </remarks>
   </Docs>
   <Members>
     <Member MemberName=".ctor">