Update documentation of :dedent: with note about leading space

Include use case for :dedent: 0 and note that for a directive without
any option whitespace is stripped by the ReST parser.

doc/usage/restructuredtext/directives.rst

@@ -598,10 +598,23 @@ __ https://pygments.org/docs/lexers
       are removed via :func:`textwrap.dedent()`.  For example::
 
          .. code-block:: ruby
+            :linenos:
             :dedent: 4
 
                 some ruby code
 
+      Using ``0`` as argument does not strip anything and can be used to
+      preserve leading whitespace in the code.  Without any option to
+      a directive, leading whitespace would be treated as part of the indent of
+      the content of the directive and not as part of the content of the
+      literal block (:duref:`ref <indentation>`) and thus will not occur in the
+      output::
+
+         .. code-block:: ruby
+            :dedent: 0
+
+                some ruby code with 4 leading spaces in the output
+
       .. versionadded:: 1.3
       .. versionchanged:: 3.5
          Support automatic dedent.