more consistent and RST-ish inline-markup -- starting with vcl.rst

in particular

* VCL language and other literals as ``literal``
* placeholders and emphasis as *emphasis*
* no `interpreted text` except where it actually *is* that

Ref http://docutils.sourceforge.net/docs/ref/rst/restructuredtext.html#character-level-inline-markup

doc/README.WRITING_RST.rst

 THINGS TO CONSIDER WHEN WRITING VARNISH RST DOCUMENTATION
 =========================================================
 
+Inline Markup
+-------------
+
+Please try to be consistent with inline markup and fix places which do
+not follow the style:
+
+* VCL language and other literals as ``literal``
+
+* placeholders and emphasis as *emphasis*
+
+* no `interpreted text` except where it actually *is* that
+
+.. _Reference: http://docutils.sourceforge.net/docs/ref/rst/restructuredtext.html#character-level-inline-markup
+
 References are tricky
 ---------------------