TRPL: Add "verboten" to the glossary

Maybe a more suitable resolution to the issues raised in rust-lang#19929 and rust-lang#20439.

Author: tomjakubowski

src/doc/trpl/glossary.md

@@ -63,3 +63,9 @@ themselves form part of larger expressions.
 
 In computer programming, a statement is the smallest standalone element of a
 programming language that commands a computer to perform an action.
+
+### Verboten
+
+*adjective*
+
+1. forbidden, as by law; prohibited.