Remove ordering guarantees in the MapField documentation

This doesn't currently change the ordering in the implementation, but allows us to do so in the future.
We also need to change
https://developers.google.com/protocol-buffers/docs/reference/csharp-generated#singular
which states "Finally, unlike Dictionary<TKey, TValue>, MapField<TKey, TValue> preserves insertion order of entries."
(We can just remove that sentence, I think.)

csharp/src/Google.Protobuf/Collections/MapField.cs

@@ -48,9 +48,6 @@ namespace Google.Protobuf.Collections
     /// <typeparam name="TValue">Value type in the map. Must be a type supported by Protocol Buffers.</typeparam>
     /// <remarks>
     /// <para>
-    /// This implementation preserves insertion order for simplicity of testing
-    /// code using maps fields. Overwriting an existing entry does not change the
-    /// position of that entry within the map. Equality is not order-sensitive.
     /// For string keys, the equality comparison is provided by <see cref="StringComparer.Ordinal" />.
     /// </para>
     /// <para>
@@ -65,6 +62,10 @@ namespace Google.Protobuf.Collections
     /// supported by Protocol Buffers (e.g. using a key type of <code>byte</code>) but nor does it guarantee
     /// that all operations will work in such cases.
     /// </para>
+    /// <para>
+    /// The order in which entries are returned when iterating over this object is undefined, and may change
+    /// in future versions.
+    /// </para>
     /// </remarks>
     public sealed class MapField<TKey, TValue> : IDeepCloneable<MapField<TKey, TValue>>, IDictionary<TKey, TValue>, IEquatable<MapField<TKey, TValue>>, IDictionary
     {