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/email.errors.rst b/Doc/library/email.errors.rst
index 511ad16..f4b9f52 100644
--- a/Doc/library/email.errors.rst
+++ b/Doc/library/email.errors.rst
@@ -99,7 +99,7 @@
 * :class:`MultipartInvariantViolationDefect` -- A message claimed to be a
   :mimetype:`multipart`, but no subparts were found.  Note that when a message
   has this defect, its :meth:`~email.message.Message.is_multipart` method may
-  return false even though its content type claims to be :mimetype:`multipart`.
+  return ``False`` even though its content type claims to be :mimetype:`multipart`.
 
 * :class:`InvalidBase64PaddingDefect` -- When decoding a block of base64
   encoded bytes, the padding was not correct.  Enough padding is added to