Document the CLDR↔GlotPress slug mismatch in the plural-rules generator

`generate_ios_plural_rules.rb` keys CLDR locale codes against GlotPress
slugs 1:1, so a language GlotPress addresses by a different code than CLDR
(Belarusian is `bel` in GlotPress vs `be` in CLDR; likewise `mya`/`my`,
`dzo`/`dz`, …) returns `nil` from `nplurals_for` and is omitted from the
generated table. Those locales fail loud (UnknownLocaleError, now a clean
`UI.user_error!`) rather than being mis-mapped, and none are in the apps'
shipped set — but the omission was undocumented and reads as accidental,
since the old hand-written table had `be`.

Note the limitation at `nplurals_for` and in the header, including the fix
(a vetted CLDR→GlotPress alias map — a literal mapping, not prefix-matching,
since `sc`/`scn` and `ve`/`vec` are different languages) for if one of these
locales ever ships. Comment-only; the generated table is unchanged.

Author: jkmassel

rakelib/generate_ios_plural_rules.rb

@@ -16,6 +16,10 @@
 #                         6 categories) the locale is omitted on purpose — the
 #                         converter then fails loud rather than guessing.
 #
+# A locale is also omitted when GlotPress addresses it under a different slug
+# than CLDR (e.g. Belarusian is `bel` in GlotPress vs `be` in CLDR) — see
+# `nplurals_for`. These too fail loud rather than mis-map; none are in SHIPPED.
+#
 # Inputs are vendored under rakelib/plural_rules_data/ for offline reproducibility:
 #   - cldr_plurals.xml        — Unicode CLDR common/supplemental/plurals.xml
 #   - glotpress_nplurals.json — { slug => nplurals } from GlotPress GP_Locales
@@ -65,6 +69,16 @@ def glotpress_nplurals
   JSON.parse(File.read(File.join(DATA_DIR, 'glotpress_nplurals.json')))
 end
 
+# GlotPress's plural-form count for a CLDR locale code. CLDR and GlotPress
+# usually agree on slugs, but not always: GlotPress addresses some languages by
+# an ISO-639-2/3 code where CLDR uses the 639-1 code (Belarusian is `bel` in
+# GlotPress vs `be` in CLDR; likewise `mya`/`my`, `dzo`/`dz`, `wol`/`wo`, …). We
+# only try the CLDR code and its regional variants, so those locales return
+# `nil` here and are omitted from the table — they then fail loud
+# (UnknownLocaleError) at runtime rather than being mis-mapped. None are in the
+# apps' shipped set today; to support one, add a vetted CLDR→GlotPress alias
+# here — a literal mapping, NOT a prefix match (`sc`→`scn` is Sardinian vs
+# Sicilian, `ve`→`vec` is Venda vs Venetian, so prefix-guessing is unsafe).
 def nplurals_for(base, glotpress)
   glotpress[base] || glotpress[glotpress.keys.sort.find { |slug| slug.start_with?("#{base}-") }]
 end