Audit reference manual: 3.2 Special unicode productions

Mention non_ascii_idents feature gate and remove unused productions

Author: Robin Kruppe

src/doc/reference.md

@@ -103,12 +103,14 @@ explicit codepoint lists. [^inputformat]
 ## Special Unicode Productions
 
 The following productions in the Rust grammar are defined in terms of Unicode
-properties: `ident`, `non_null`, `non_star`, `non_eol`, `non_slash_or_star`,
-`non_single_quote` and `non_double_quote`.
+properties: `ident`, `non_null`, `non_eol`, `non_single_quote` and `non_double_quote`.
 
 ### Identifiers
 
-The `ident` production is any nonempty Unicode string of the following form:
+The `ident` production is any nonempty Unicode[^non_ascii_idents] string of the following form:
+
+[^non_ascii_idents]: Non-ASCII characters in identifiers are currently feature
+  gated. This is expected to improve soon.
 
 - The first character has property `XID_start`
 - The remaining characters have property `XID_continue`
@@ -125,8 +127,6 @@ Some productions are defined by exclusion of particular Unicode characters:
 
 - `non_null` is any single Unicode character aside from `U+0000` (null)
 - `non_eol` is `non_null` restricted to exclude `U+000A` (`'\n'`)
-- `non_star` is `non_null` restricted to exclude `U+002A` (`*`)
-- `non_slash_or_star` is `non_null` restricted to exclude `U+002F` (`/`) and `U+002A` (`*`)
 - `non_single_quote` is `non_null` restricted to exclude `U+0027`  (`'`)
 - `non_double_quote` is `non_null` restricted to exclude `U+0022` (`"`)