padata: Add a reference to the api documentation

Add a reference to the padata api documentation at Documentation/padata.txt
Suggested-by: NPeter Zijlstra <peterz@infradead.org>
Signed-off-by: NSteffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

kernel/padata.c

 /*
  * padata.c - generic interface to process data streams in parallel
  *
+ * See Documentation/padata.txt for an api documentation.
+ *
  * Copyright (C) 2008, 2009 secunet Security Networks AG
  * Copyright (C) 2008, 2009 Steffen Klassert <steffen.klassert@secunet.com>
  *