#997912: acknowledge nested scopes in tutorial.

Doc/tutorial/controlflow.rst

@@ -204,10 +204,11 @@ it.
 The *execution* of a function introduces a new symbol table used for the local
 variables of the function.  More precisely, all variable assignments in a
 function store the value in the local symbol table; whereas variable references
-first look in the local symbol table, then in the global symbol table, and then
-in the table of built-in names. Thus,  global variables cannot be directly
-assigned a value within a function (unless named in a :keyword:`global`
-statement), although they may be referenced.
+first look in the local symbol table, then in the local symbol tables of
+enclosing functions, then in the global symbol table, and finally in the table
+of built-in names. Thus, global variables cannot be directly assigned a value
+within a function (unless named in a :keyword:`global` statement), although they
+may be referenced.
 
 The actual parameters (arguments) to a function call are introduced in the local
 symbol table of the called function when it is called; thus, arguments are