For-else deserves its own section in the tutorial (pythonGH-123946)

* For-else deserves its own section in the tutorial

* remove mention of unrolling the loop

* Update Doc/tutorial/controlflow.rst

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

---------

(cherry picked from commit ffdc80e)

Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

Doc/tutorial/controlflow.rst

@@ -160,16 +160,52 @@ arguments. In chapter :ref:`tut-structures`, we will discuss in more detail abo
 
 .. _tut-break:
 
-:keyword:`!break` and :keyword:`!continue` Statements, and :keyword:`!else` Clauses on Loops
-============================================================================================
+:keyword:`!break` and :keyword:`!continue` Statements
+=====================================================
 
 The :keyword:`break` statement breaks out of the innermost enclosing
-:keyword:`for` or :keyword:`while` loop.
+:keyword:`for` or :keyword:`while` loop::
 
-A :keyword:`!for` or :keyword:`!while` loop can include an :keyword:`!else` clause.
+ >>> for n in range(2, 10):
+ ... for x in range(2, n):
+ ... if n % x == 0:
+ ... print(f"{n} equals {x} * {n//x}")
+ ... break
+ ...
+ 4 equals 2 * 2
+ 6 equals 2 * 3
+ 8 equals 2 * 4
+ 9 equals 3 * 3
+
+The :keyword:`continue` statement continues with the next
+iteration of the loop::
+
+ >>> for num in range(2, 10):
+ ... if num % 2 == 0:
+ ... print(f"Found an even number {num}")
+ ... continue
+ ... print(f"Found an odd number {num}")
+ ...
+ Found an even number 2
+ Found an odd number 3
+ Found an even number 4
+ Found an odd number 5
+ Found an even number 6
+ Found an odd number 7
+ Found an even number 8
+ Found an odd number 9
+
+.. _tut-for-else:
+
+:keyword:`!else` Clauses on Loops
+=================================
+
+In a :keyword:`!for` or :keyword:`!while` loop the :keyword:`!break` statement
+may be paired with an :keyword:`!else` clause. If the loop finishes without
+executing the :keyword:`!break`, the :keyword:`!else` clause executes.
 
 In a :keyword:`for` loop, the :keyword:`!else` clause is executed
-after the loop reaches its final iteration.
+after the loop finishes its final iteration, that is, if no break occurred.
 
 In a :keyword:`while` loop, it's executed after the loop's condition becomes false.
 
@@ -198,32 +234,19 @@ which searches for prime numbers::
  9 equals 3 * 3
 
 (Yes, this is the correct code. Look closely: the ``else`` clause belongs to
-the :keyword:`for` loop, **not** the :keyword:`if` statement.)
-
-When used with a loop, the ``else`` clause has more in common with the
-``else`` clause of a :keyword:`try` statement than it does with that of
-:keyword:`if` statements: a :keyword:`try` statement's ``else`` clause runs
-when no exception occurs, and a loop's ``else`` clause runs when no ``break``
-occurs. For more on the :keyword:`!try` statement and exceptions, see
-:ref:`tut-handling`.
-
-The :keyword:`continue` statement, also borrowed from C, continues with the next
-iteration of the loop::
-
- >>> for num in range(2, 10):
- ... if num % 2 == 0:
- ... print("Found an even number", num)
- ... continue
- ... print("Found an odd number", num)
- ...
- Found an even number 2
- Found an odd number 3
- Found an even number 4
- Found an odd number 5
- Found an even number 6
- Found an odd number 7
- Found an even number 8
- Found an odd number 9
+the ``for`` loop, **not** the ``if`` statement.)
+
+One way to think of the else clause is to imagine it paired with the ``if``
+inside the loop. As the loop executes, it will run a sequence like
+if/if/if/else. The ``if`` is inside the loop, encountered a number of times. If
+the condition is ever true, a ``break`` will happen. If the condition is never
+true, the ``else`` clause outside the loop will execute.
+
+When used with a loop, the ``else`` clause has more in common with the ``else``
+clause of a :keyword:`try` statement than it does with that of ``if``
+statements: a ``try`` statement's ``else`` clause runs when no exception
+occurs, and a loop's ``else`` clause runs when no ``break`` occurs. For more on
+the ``try`` statement and exceptions, see :ref:`tut-handling`.
 
 .. _tut-pass: