Clarify what happens when you add existing key to Map

fixes dotnet/fsharp#524
closes dotnet/fsharp#525

Author: svick

src/fsharp/FSharp.Core/map.fsi

@@ -16,7 +16,8 @@ namespace Microsoft.FSharp.Collections
     [<CompiledName("FSharpMap`2")>]
     [<Sealed>]
     type Map<[<EqualityConditionalOn>]'Key,[<EqualityConditionalOn;ComparisonConditionalOn>]'Value when 'Key : comparison> =
-        /// <summary>Returns a new map with the binding added to the given map.</summary>
+        /// <summary>Returns a new map with the binding added to the given map.
+        /// If a binding with the given key already exists in the input map, the existing binding is replaced by the new binding in the result map.</summary>
         /// <param name="key">The input key.</param>
         /// <returns>The resulting map.</returns>
         member Add: key:'Key * value:'Value -> Map<'Key,'Value>
@@ -67,7 +68,8 @@ namespace Microsoft.FSharp.Collections
     [<RequireQualifiedAccess>]
     module Map = 
 
-        /// <summary>Returns a new map with the binding added to the given map.</summary>
+        /// <summary>Returns a new map with the binding added to the given map.
+        /// If a binding with the given key already exists in the input map, the existing binding is replaced by the new binding in the result map.</summary>
         /// <param name="key">The input key.</param>
         /// <param name="value">The input value.</param>
         /// <param name="table">The input map.</param>