Improve ConcurrentDictionary.TryUpdate (dotnet#163)

Improve the summary of ConcurrentyDictionary.TryUpdate. The summary mentions parameters in a different order and with different names than they are represented in the method signature. This is confusing:
![image](https://user-images.githubusercontent.com/2090172/39812540-579f1164-538d-11e8-8e2d-cb58b4508a66.png)

To clear things up, I've rephrased the summary to mention parameters in the same order as they occur in the method signature and used <paramref name="XYZ"/> to reference them.

Note: I'm "cross-PR-ing" this from dotnet/corefx#29599
Though I'm wondering a bit about the duplication of the api-doc.
Will we really have to maintain the code-doc twice? Will one become obsolete? Will they be merged (one-way or two-way) automagically?
I haven't found anything on that topic in the contribution guide. But then I admit I didn't read every word of it, either...

Author: BrunoJuchli

xml/System.Collections.Concurrent/ConcurrentDictionary`2.xml

@@ -1994,7 +1994,7 @@
         <param name="key">The key whose value is compared with <c>comparisonValue</c> and possibly replaced.</param>
         <param name="newValue">The value that replaces the value of the element that has the specified <c>key</c> if the comparison results in equality.</param>
         <param name="comparisonValue">The value that is compared to the value of the element that has the specified <c>key</c>.</param>
-        <summary>Compares the existing value for the specified key with a specified value, and if they are equal, updates the key with a third value.</summary>
+        <summary>Updates the value associated with <paramref name="key" /> to <paramref name="newValue" /> if the existing value is equal to <paramref name="comparisonValue" />.</summary>
         <returns>
           <see langword="true" /> if the value with <paramref name="key" /> was equal to <paramref name="comparisonValue" /> and was replaced with <paramref name="newValue" />; otherwise, <see langword="false" />.</returns>
         <remarks>
@@ -2047,4 +2047,4 @@
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>