doc-guide: kernel-doc: add %CONST examples

Add examples of using '%' for formatting constant values to
facilitate more usage of "%CONST" in kernel-doc.

Suggested-by: Tony Luck <[e-mail]>
Signed-off-by: Randy Dunlap <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Message-ID: <[e-mail]>

Documentation/doc-guide/kernel-doc.rst

···
 ``%CONST``
   Name of a constant. (No cross-referencing, just formatting.)
 
+  Examples::
+
+    %0    %NULL    %-1    %-EFAULT    %-EINVAL    %-ENOMEM
+
 ````literal````
   A literal block that should be handled as-is. The output will use a
   ``monospaced font``.