XML documentation for IArgument interface (#1101)

* Add XML documentation to the IArgument interface

* Fix documentation. Interface only has getter.

* Make consistent with other interfaces, based on recommended standards

Author: pieter-venter

src/System.CommandLine/IArgument.cs

@@ -5,10 +5,16 @@
 
 namespace System.CommandLine
 {
+    /// <summary>
+    /// Defines a symbol with an arity.
+    /// </summary>
     public interface IArgument : 
         ISymbol,
         IValueDescriptor
     {
+        /// <summary>
+        /// Gets the arity of the argument.
+        /// </summary>
         IArgumentArity Arity { get; }
     }
 }