Moved details of unstable non-ascii identifiers from the Reference to the Unstable book

Author: brauliobz

src/doc/unstable-book/src/language-features/non-ascii-idents.md

@@ -15,4 +15,34 @@ The `non_ascii_idents` feature adds support for non-ASCII identifiers.
 
 const ε: f64 = 0.00001f64;
 const Π: f64 = 3.14f64;
-```
+```
+
+## Changes to the language reference
+
+> **<sup>Lexer:<sup>**  
+> IDENTIFIER :  
+> &nbsp;&nbsp; &nbsp;&nbsp; XID_start XID_continue<sup>\*</sup>  
+> &nbsp;&nbsp; | `_` XID_continue<sup>+</sup>  
+
+An identifier is any nonempty Unicode string of the following form:
+
+Either
+
+   * The first character has property [`XID_start`]
+   * The remaining characters have property [`XID_continue`]
+
+Or
+
+   * The first character is `_`
+   * The identifier is more than one character, `_` alone is not an identifier
+   * The remaining characters have property [`XID_continue`]
+
+that does _not_ occur in the set of [strict keywords].
+
+> **Note**: [`XID_start`] and [`XID_continue`] as character properties cover the
+> character ranges used to form the more familiar C and Java language-family
+> identifiers.
+
+[`XID_start`]:  http://unicode.org/cldr/utility/list-unicodeset.jsp?a=%5B%3AXID_Start%3A%5D&abb=on&g=&i=
+[`XID_continue`]: http://unicode.org/cldr/utility/list-unicodeset.jsp?a=%5B%3AXID_Continue%3A%5D&abb=on&g=&i=
+[strict keywords]: keywords.html#strict-keywords