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/os.rst b/Doc/library/os.rst
index 48bd6b9..652d8de 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -148,7 +148,7 @@
    versa).
 
    :data:`environb` is only available if :data:`supports_bytes_environ` is
-   True.
+   ``True``.
 
    .. versionadded:: 3.2
 
@@ -236,7 +236,7 @@
    *default* if it doesn't. *key*, *default* and the result are bytes.
 
    :func:`getenvb` is only available if :data:`supports_bytes_environ`
-   is True.
+   is ``True``.
 
    .. availability:: most flavors of Unix.