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>".

Lib/tkinter/__init__.py

@@ -1053,7 +1053,7 @@ class Misc:
         return self.tk.call('winfo', 'class', self._w)
 
     def winfo_colormapfull(self):
-        """Return true if at the last color request the colormap was full."""
+        """Return True if at the last color request the colormap was full."""
         return self.tk.getboolean(
             self.tk.call('winfo', 'colormapfull', self._w))
 
@@ -3226,7 +3226,7 @@ class Listbox(Widget, XView, YView):
     select_clear = selection_clear
 
     def selection_includes(self, index):
-        """Return 1 if INDEX is part of the selection."""
+        """Return True if INDEX is part of the selection."""
         return self.tk.getboolean(self.tk.call(
             self._w, 'selection', 'includes', index))