CP4.2: from_tag docs name ProofLang::Pack

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FypgCX8LsYYE8fsj4byBm2

crates/proof/src/lang.rs

@@ -106,7 +106,8 @@ impl ProofLang {
     /// The proofing language of a BCP 47 tag (`pl-PL`, `en_GB`, `PL`…). The primary subtag
     /// counts, and for English and Spanish the region too ([`ProofLang::english`],
     /// [`ProofLang::spanish`]). No tag (or an empty one) is English, WordCraft's default editing
-    /// language.
+    /// language. Any other valid language tag is [`ProofLang::Pack`] with its [`PackLocale`];
+    /// `x-none`, `und`, `zxx`, `mul`, `mis` and malformed tags are [`ProofLang::Other`].
     pub fn from_tag(tag: Option<&str>) -> ProofLang {
         let Some(tag) = tag.map(str::trim).filter(|t| !t.is_empty()) else { return ProofLang::En };
         let primary = tag.split(['-', '_']).next().unwrap_or("");