Add a Lexer rules block for reserved prefixes

Author: mattheww

src/tokens.md

@@ -661,6 +661,11 @@ them are referred to as "token trees" in [macros].  The three types of brackets
 
 ## Reserved prefixes
 
+> **<sup>Lexer 2018+</sup>**\
+> RESERVED_TOKEN_DOUBLE_QUOTE : ( IDENTIFIER_OR_KEYWORD <sub>_Except `b` or `r` or `br`_</sub> | `_` ) `"`\
+> RESERVED_TOKEN_SINGLE_QUOTE : ( IDENTIFIER_OR_KEYWORD <sub>_Except `b`_</sub> | `_` ) `'`\
+> RESERVED_TOKEN_POUND : ( IDENTIFIER_OR_KEYWORD <sub>_Except `r` or `br`_</sub> | `_` ) `#`
+
 Some lexical forms known as _reserved prefixes_ are reserved for future use.
 
 Source input which would otherwise be lexically interpreted as a non-raw identifier (or a keyword or `_`) which is immediately followed by a `#`, `'`, or `"` character (without intervening whitespace) is identified as a reserved prefix.