Fix FromBase64Transform.CanTransformMultipleBlocks to not say "always false". (#11511)

Author: vcsjones

xml/System.Security.Cryptography/FromBase64Transform.xml

@@ -290,12 +290,14 @@
       </ReturnValue>
       <Docs>
         <summary>Gets a value that indicates whether multiple blocks can be transformed.</summary>
-        <value>Always <see langword="false" />.</value>
+        <value>A value that indicates whether multiple blocks can be transformed.</value>
         <remarks>
           <format type="text/markdown"><![CDATA[
 
 ## Remarks
- In the current implementation, only one block can be transformed at a time, so this property is always `false`.
+ Prior to .NET 5, only one block can be transformed at a time, so this property returns `false`.
+
+ Starting in .NET 5, multiple blocks can be transformed, so this property returns `true`.
 
  ]]></format>
         </remarks>