docs: record that a map source's unavailableReason may be a function

The default map sources word their reason when it is shown, so it follows
the locale; hosts that read it directly need to know the new form.

changelog.d/0000-vue-and-pinia.md

@@ -6,6 +6,11 @@
   unsaved-work prompt are removed, since a switch no longer loses anything
   (samehkhamis, #0000).
 
+- A map source's `unavailableReason` may be a function, read when the reason
+  is shown so it follows the locale; the sources from `createDefaultMapSources`
+  (`./maps/defaults`) give functions. `MapSourceController` reads either form
+  (samehkhamis, #0000).
+
 - The application views translate their text with `t()` as they render,
   so it follows the active locale; they no longer carry `data-i18n*`
   attributes, and `npm run i18n` rejects those in a Vue template. The boot