Address #619: add a note about * variant (#666)

* Address #619: add a note about `*` variant

Per my action-item, add an appropriate note about the requirement for the fallback variant, including text seeking feedback in tech preview.

* Address @stasm's comment

Author: aphillips

spec/formatting.md

@@ -372,6 +372,30 @@ to provide the _pattern_ for the formatting operation.
 This is done by ordering and filtering the available _variant_ statements
 according to their _key_ values and selecting the first one.
 
+> [!NOTE]
+> At least one _variant_ is required to have all of its _keys_ consist of
+> the fallback value `*`.
+> Some _selectors_ might be implemented in a way that the key value `*`
+> cannot be selected in a _valid_ _message_.
+> In other cases, this key value might be unreachable only in certain locales.
+> This could result in the need in some locales to create
+> one or more _variants_ that do not make sense grammatically for that language.
+> > For example, in the `pl` (Polish) locale, this _message_ cannot reach
+> > the `*` _variant_:
+> > ```
+> > .match {$num :integer}
+> > 0    {{ }}
+> > one  {{ }}
+> > few  {{ }}
+> > many {{ }}
+> > *    {{Only used by fractions in Polish.}}
+> > ```
+>
+> In the Tech Preview, feedback from users and implementers is desired about
+> whether to relax the requirement that such a "fallback _variant_" appear in
+> every message, versus the potential for a _message_ to fail at runtime
+> because no matching _variant_ is available.
+
 The number of _keys_ in each _variant_ MUST equal the number of _selectors_.
 
 Each _key_ corresponds to a _selector_ by its position in the _variant_.