change metadata (#652)

(cherry picked from commit e1ce073)

Author: shuangela

source/fundamentals/authentication.txt

@@ -16,6 +16,7 @@ Authentication Mechanisms
 
 .. meta::
    :keywords: .NET, community, security
+   :description: Learn how to authenticate to MongoDB using various authentication mechanisms in the .NET/C# Driver compatible with different MongoDB Server editions.
 
 .. toctree::
    :caption: Authentication