[DO NOT MERGE] Add remark to ICryptoTransform.TransformBlock (dotnet#2096)

Add remark to ICryptoTransform.TransformBlock

Author: guardrex

xml/System.Security.Cryptography/ICryptoTransform.xml

@@ -183,7 +183,14 @@
         <param name="outputOffset">The offset into the output byte array from which to begin writing data.</param>
         <summary>Transforms the specified region of the input byte array and copies the resulting transform to the specified region of the output byte array.</summary>
         <returns>The number of bytes written.</returns>
-        <remarks>To be added.</remarks>
+        <remarks>
+          <format type="text/markdown"><![CDATA[  
+  
+## Remarks  
+The return value of <xref:System.Security.Cryptography.ICryptoTransform.TransformBlock%2A> is the number of bytes returned to `outputBuffer` and is always less than or equal to <xref:System.Security.Cryptography.ICryptoTransform.OutputBlockSize>. If <xref:System.Security.Cryptography.ICryptoTransform.CanTransformMultipleBlocks> is `true`, then `inputCount` must be any positive multiple of <xref:System.Security.Cryptography.ICryptoTransform.InputBlockSize>.
+  
+ ]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="TransformFinalBlock">