Say that _#, _', and _" are reserved prefixes

Author: mattheww

src/tokens.md

@@ -663,7 +663,7 @@ them are referred to as "token trees" in [macros].  The three types of brackets
 
 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) which is immediately followed by a `#`, `'`, or `"` character (without intervening whitespace) is identified as a reserved prefix.
+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.
 
 Note that raw identifiers, raw string literals, and raw byte string literals may contain a `#` character but are not interpreted as containing a reserved prefix.