docs(lang): sync issues/0089 member-name exemption with the f32/f64 keyword caveat [F0.6]

issues/0089-backtick-raw-identifier.md

@@ -67,6 +67,15 @@
 >      ([src/ir/semantic_diagnostics.zig]) check only the *type* name (and method
 >      *params*), not field / tag / variant / method-signature names. The backtick
 >      is optional there (`obj.s2` and `` obj.`s2 `` resolve to the same member).
+>      This bare member-name exemption covers only the **identifier-classified**
+>      reserved spellings — `s1`..`s64`, `u1`..`u64`, `bool`, `string`, `void`,
+>      `usize`, `isize`, `Any` — which all lex as ordinary identifiers. The two
+>      **keyword-classified** spellings, `f32` and `f64`, are lexer keywords
+>      ([src/token.zig]), and a member-name slot requires an identifier token
+>      ([src/parser.zig]); a bare `f32` / `f64` is therefore rejected at parse
+>      (`expected field name in struct`) even in a member position, and still needs
+>      the backtick there too — `` struct { `f32: s64; } `` / `` union { `f64: … } ``
+>      / `` protocol { `f32 :: (self); } `` work as field / tag / method names.
 >      The exemption stops at member *definitions*: an `impl` method is a real
 >      function reached through the `impl_block` → `fn_decl` arm, so a
 >      reserved-spelled impl method needs the backtick (`` `s2 :: (self) ``), no