bpo-38738: Fix formatting of True and False. (GH-17083)

* "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>".
diff --git a/Doc/library/difflib.rst b/Doc/library/difflib.rst
index c2a19dc..ada311b 100644
--- a/Doc/library/difflib.rst
+++ b/Doc/library/difflib.rst
@@ -338,14 +338,14 @@
 
 .. function:: IS_LINE_JUNK(line)
 
-   Return true for ignorable lines.  The line *line* is ignorable if *line* is
+   Return ``True`` for ignorable lines.  The line *line* is ignorable if *line* is
    blank or contains a single ``'#'``, otherwise it is not ignorable.  Used as a
    default for parameter *linejunk* in :func:`ndiff` in older versions.
 
 
 .. function:: IS_CHARACTER_JUNK(ch)
 
-   Return true for ignorable characters.  The character *ch* is ignorable if *ch*
+   Return ``True`` for ignorable characters.  The character *ch* is ignorable if *ch*
    is a space or tab, otherwise it is not ignorable.  Used as a default for
    parameter *charjunk* in :func:`ndiff`.
 
@@ -370,7 +370,7 @@
    Optional argument *isjunk* must be ``None`` (the default) or a one-argument
    function that takes a sequence element and returns true if and only if the
    element is "junk" and should be ignored. Passing ``None`` for *isjunk* is
-   equivalent to passing ``lambda x: 0``; in other words, no elements are ignored.
+   equivalent to passing ``lambda x: False``; in other words, no elements are ignored.
    For example, pass::
 
       lambda x: x in " \t"