[C# Analyzer] Configuration Versioning Note (#81)

* versioning note

* option only

* RR feedback

Author: rachel-mack

source/configuration.txt

@@ -114,6 +114,10 @@ by the {+product-short+}:
          its nested namespaces only
        - ``"None"``: Do not analyze LINQ expressions
 
+       .. important::
+
+          The ``LinqAnalysisVerbosity`` configuration option is available only in {+product-short+} v2.0 and later.
+       
        | **Accepted Values**: ``"All"``, ``"Medium"``, or ``"None"``
        | **Default**: ``"Medium"``