Add ReadTimeout unit on SslStream (dotnet#2978)

The ReadTimeout is expressed in milliseconds according to my tests, however, this is not stated in the documentation. I used the documentation for the Timer.Interval property as a guideline to add unit information to the ReadTimeout on the SslStream.

Author: Crown0815

xml/System.Net.Security/SslStream.xml

@@ -3775,8 +3775,8 @@ This property gets the cipher suite that is going to be used in the communicatio
         <ReturnType>System.Int32</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets or sets the amount of time a read operation blocks waiting for data.</summary>
-        <value>The amount of time that elapses before a synchronous read operation fails.</value>
+        <summary>Gets or sets the amount of time, expressed in milliseconds, a read operation blocks waiting for data.</summary>
+        <value>The amount of time, in milliseconds, that elapses before a synchronous read operation fails.</value>
         <remarks>
           <format type="text/markdown"><![CDATA[  
   
@@ -4373,4 +4373,4 @@ The <xref:System.Net.Security.SslStream> class does not support multiple simulta
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>