gh-106318: Add example for str.isascii() (GH-137558)

(cherry picked from commit 0afcb51)

Co-authored-by: Adorilson Bezerra <adorilson@gmail.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Author: 3 people

Doc/library/stdtypes.rst

@@ -1927,7 +1927,14 @@ expression support in the :mod:`re` module).
 
    Return ``True`` if the string is empty or all characters in the string are ASCII,
    ``False`` otherwise.
-   ASCII characters have code points in the range U+0000-U+007F.
+   ASCII characters have code points in the range U+0000-U+007F. For example:
+
+   .. doctest::
+
+      >>> 'ASCII characters'.isascii()
+      True
+      >>> 'µ'.isascii()
+      False
 
    .. versionadded:: 3.7