Added grammar for in-out-expression.

Author: bjlanier

ReferenceManual/Expressions.rst

@@ -68,6 +68,8 @@ see :doc:`../LanguageGuide/BasicOperators` and :doc:`../LanguageGuide/AdvancedOp
     Grammar of a prefix expression
 
     prefix-expression --> prefix-operators-OPT postfix-expression
+    prefix-expression --> in-out-expression
+    in-out-expression --> ``&`` identifier
     prefix-operators --> prefix-operator prefix-operators-OPT