Add docs to Map

distribution/lib/Standard/Base/0.0.0-dev/src/Data/Map.enso

@@ -15,6 +15,12 @@ from project import Error, Nothing, Any, Panic
    for equality any objects that can appear in Enso - primitives, Atoms, values coming from
    different languages, etc.
 
+   For keys that are not reflexive, like `Number.nan`, "Same Value Equality", from JavaScript
+   specification in https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness#same-value-zero_equality,
+   is used. This means that both `Number.nan` and types with comparators that violate
+   reflexivity (e.g. their `compare` method always returns `Nothing`) can be used as keys
+   in the Map.
+
    A single key-value pair is called an *entry*.
 
    It is possible to pass a Map created in Enso to foreign functions, where it will be treated