Key sizes can only be certain lengths (#3992)

* Key sizes can only be certain lengths

The wording could be interpreted as "any value between 128 and 256 bits is valid" leading to confision.
Specifying exact values will hopefully provide more clarity and eliminate confusion.
See https://github.com/microsoft/referencesource/blob/master/System.Core/System/Security/Cryptography/AesCryptoServiceProvider.cs#L323 for implementation details.

* Update AesCng.xml

added same valid `KeySize` clarification to related `AesCng` class

* Update AesCng.xml

code review changes

Author: tkhadimullin

xml/System.Security.Cryptography/AesCng.xml

@@ -645,7 +645,9 @@
         <remarks>
           <format type="text/markdown"><![CDATA[  
   
-## Remarks  
+## Remarks 
+ For AES, the legal key sizes are 128, 192, and 256 bits.
+
  If you've created the <xref:System.Security.Cryptography.AesCng> object using an existing persisted key and you set the value of the <xref:System.Security.Cryptography.AesCng.KeySize%2A> property,  the persisted key will no longer be used and an ephemeral key will be used instead. If you need to use the persisted key again, a new instance of <xref:System.Security.Cryptography.AesCng> class needs to be created.  
   
  ]]></format>

xml/System.Security.Cryptography/AesCryptoServiceProvider.xml

@@ -693,7 +693,7 @@
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- The minimum size of the key is 128 bits, and the maximum size is 256 bits.  
+ For AES, the legal key sizes are 128, 192, and 256 bits.
   
  Changing the `KeySize` value resets the key and generates a new random key. This happens whenever the `KeySize` property setter is invoked (including when it's assigned the same value).