DOC: Extend explanation on single vs double backticks.

Add the special case for constant, and try to separate semantic (can
link), to style (monospace, link)

See also numpy#525

doc/format.rst

@@ -753,18 +753,26 @@ monospaced font; in reST, *double* backticks are for ``monospaced font``,
 whereas the behavior of single backticks is defined by the default role. This
 leads to the following style recommendations:
 
-- Module, class, function, method, and attribute names should render as
- hyperlinks in monospaced font (e.g. :any:`numpy`); depending on project
- settings, this may be accomplished simply be enclosing them in single
- backticks. If the hyperlink does not render as intended, explicitly
- include the appropriate role and/or namespace.
+- Any object that can be linked to should be enclosed in single backticks. That
+ is to say : Module, class, function, method, and attribute names are some of
+ the objects that should be enclosed in single backticks. On a case by cases,
+ instances may also be included in single backticks, like :py:obj:`None`,
+ :py:obj:`True`, :py:obj:`False`.
+
+ These will typically render as hyperlinks in monospaced font (e.g.
+ :any:`numpy`) to the relevant object documentation; depending on project
+ settings. If the hyperlink does not render properly or point to the intended
+ object, explicitly include the appropriate role and/or namespace.
+
 - This guide continues to recommended that parameter names be enclosed within
  single backticks. Currently, this may cause parameter names to render
  improperly and cause warnings, but numpydoc will soon release a feature
  that causes them to render as monospaced hyperlinks to the parameter
  documentation.
-- All other text that is intended to render in ``monospaced`` font should be
- enclosed within ````double backticks````.
+
+- All other text that is intended to render in ``monospaced`` font and is not
+ supposed to refer to another docs page should be enclosed within ````double
+ backticks````.
 
 A more extensive example of reST markup can be found in `this example
 document <http://docutils.sourceforge.net/docs/user/rst/demo.txt>`_;