Docs: use boolean constants for returning boolean value (GH-133325)

Doc/library/stdtypes.rst

@@ -2012,7 +2012,7 @@ expression support in the :mod:`re` module).
 
 .. method:: str.isprintable()
 
-   Return true if all characters in the string are printable, false if it
+   Return ``True`` if all characters in the string are printable, ``False`` if it
    contains at least one non-printable character.
 
    Here "printable" means the character is suitable for :func:`repr` to use in

Doc/library/string.rst

@@ -858,7 +858,7 @@ these rules.  The methods of :class:`Template` are:
 
    .. method:: is_valid()
 
-      Returns false if the template has invalid placeholders that will cause
+      Returns ``False`` if the template has invalid placeholders that will cause
       :meth:`substitute` to raise :exc:`ValueError`.
 
       .. versionadded:: 3.11