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/gc.rst b/Doc/library/gc.rst
index 366861c..13eda91 100644
--- a/Doc/library/gc.rst
+++ b/Doc/library/gc.rst
@@ -35,7 +35,7 @@
 
 .. function:: isenabled()
 
-   Returns true if automatic collection is enabled.
+   Return ``True`` if automatic collection is enabled.
 
 
 .. function:: collect(generation=2)