docs: deprecate kernel-doc-nano-HOWTO.txt

Now that the new Sphinx world order is taking over, the information in
kernel-doc-nano-HOWTO.txt is outmoded.  I hate to remove it altogether,
since it's one of those files that people expect to find.  But we can add a
warning and fix all the other pointers to it.

Reminded-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle
index 9a70ddd..a096836 100644
--- a/Documentation/CodingStyle
+++ b/Documentation/CodingStyle
@@ -458,7 +458,7 @@
 it.
 
 When commenting the kernel API functions, please use the kernel-doc format.
-See the files Documentation/kernel-doc-nano-HOWTO.txt and scripts/kernel-doc
+See the files Documentation/kernel-documentation.rst and scripts/kernel-doc
 for details.
 
 Linux style for comments is the C89 "/* ... */" style.