gh-106318: Some improvements to str.isalnum() example

Author: adorilson

Doc/library/stdtypes.rst

@@ -1978,19 +1978,20 @@ expression support in the :mod:`re` module).
    Return ``True`` if all characters in the string are alphanumeric and there is at
    least one character, ``False`` otherwise.  A character ``c`` is alphanumeric if one
    of the following returns ``True``: ``c.isalpha()``, ``c.isdecimal()``,
-   ``c.isdigit()``, or ``c.isnumeric()``.
+   ``c.isdigit()``, or ``c.isnumeric()``. For example::
 
-   For example::
+   .. doctest::
 
-      >>> ''.isalnum()
-      False
       >>> 'abc123'.isalnum()
       True
       >>> 'abc123!@#'.isalnum()
       False
+      >>> ''.isalnum()
+      False
       >>> ' '.isalnum()
       False
 
+
 .. method:: str.isalpha()
 
    Return ``True`` if all characters in the string are alphabetic and there is at least