splice: completely document external interface with kerneldoc

Also add fs/splice.c as a kerneldoc target with a smaller blurb that
should be expanded to better explain the overview of splice.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

Documentation/DocBook/kernel-api.tmpl

@@ -643,4 +643,15 @@ X!Idrivers/video/console/fonts.c
 !Edrivers/spi/spi.c
   </chapter>
 
+  <chapter id="splice">
+      <title>splice API</title>
+  <para>)
+	splice is a method for moving blocks of data around inside the
+	kernel, without continually transferring it between the kernel
+	and user space.
+  </para>
+!Iinclude/linux/splice.h
+!Ffs/splice.c
+  </chapter>
+
 </book>