kernel-doc: describe the ``literal`` syntax

changeset b97f193a ("scripts/kernel-doc: fix parser
for apostrophes") added support for ``literal`` inside
kernel-doc, in order to allow using the "%" symbol inside
a literal block, as this is used at printk() description.

Document it.

Fixes: b97f193a ("scripts/kernel-doc: fix parser for apostrophes")
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/doc-guide/kernel-doc.rst

@@ -149,6 +149,16 @@ Domain`_ references.
 ``%CONST``
   Name of a constant. (No cross-referencing, just formatting.)
 
+````literal````
+  A literal block that should be handled as-is. The output will use a
+  ``monospaced font``.
+
+  Useful if you need to use special characters that would otherwise have some
+  meaning either by kernel-doc script of by reStructuredText.
+
+  This is particularly useful if you need to use things like ``%ph`` inside
+  a function description.
+
 ``$ENVVAR``
   Name of an environment variable. (No cross-referencing, just formatting.)