[3.8] bpo-38738: Fix formatting of True and False. (GH-17083) (GH-17125)

* "Return true/false" is replaced with "Return ``True``/``False``"
  if the function actually returns a bool.
* Fixed formatting of some True and False literals (now in monospace).
* Replaced "True/False" with "true/false" if it can be not only bool.
* Replaced some 1/0 with True/False if it corresponds the code.
* "Returns <bool>" is replaced with "Return <bool>".
(cherry picked from commit 138ccbb02216ca086047c3139857fb44f3dab1f9)
diff --git a/Doc/library/doctest.rst b/Doc/library/doctest.rst
index 7fc471f..a77322f 100644
--- a/Doc/library/doctest.rst
+++ b/Doc/library/doctest.rst
@@ -1529,7 +1529,7 @@
 
    A class used to check the whether the actual output from a doctest example
    matches the expected output.  :class:`OutputChecker` defines two methods:
-   :meth:`check_output`, which compares a given pair of outputs, and returns true
+   :meth:`check_output`, which compares a given pair of outputs, and returns ``True``
    if they match; and :meth:`output_difference`, which returns a string describing
    the differences between two outputs.