CLDR-17760 Add note about v48 deprecation of number symbols and forma…

…t elements without numberSystem

docs/site/downloads/cldr-47.md

@@ -102,7 +102,9 @@ For a full listing, see [Delta Data](https://unicode.org/cldr/charts/dev/delta/i
 
 ## Migration
 
-- TBD
+- Number `<symbols>` elements and format elements (`<currencyFormats>`, `<decimalFormats>`, `<percentFormats>`, `<scientificFormats>`)
+  should all have a `numberSystem` attribute. In CLDR v48 such elements without a `numberSystem` attribute will be deprecated, and the
+  corresponding entries in root will be removed; these were only intended as a long-ago migration aid.
 
 ## Known Issues