bpo-34764: improve docs example of iter() with sentinel value (GH-11222)

Doc/library/functions.rst

@@ -810,13 +810,14 @@ are always available.  They are listed here in alphabetical order.
 
    See also :ref:`typeiter`.
 
-   One useful application of the second form of :func:`iter` is to read lines of
-   a file until a certain line is reached.  The following example reads a file
-   until the :meth:`~io.TextIOBase.readline` method returns an empty string::
+   One useful application of the second form of :func:`iter` is to build a
+   block-reader. For example, reading fixed-width blocks from a binary
+   database file until the end of file is reached::
 
-      with open('mydata.txt') as fp:
-          for line in iter(fp.readline, ''):
-              process_line(line)
+      from functools import partial
+      with open('mydata.db', 'rb') as f:
+          for block in iter(partial(f.read, 64), ''):
+              process_block(block)
 
 
 .. function:: len(s)

Misc/NEWS.d/next/Documentation/2018-12-23-23-52-31.bpo-34764.DwOGeT.rst

@@ -0,0 +1 @@
+Improve example of iter() with 2nd sentinel argument.