Documentation/workqueue.txt: convert to ReST markup

... and move to Documentation/core-api folder.
Signed-off-by: Silvio Fricke <silvio.fricke@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/core-api/index.rst

@@ -7,6 +7,8 @@ Kernel and driver related documentation.
 .. toctree::
    :maxdepth: 1
 
+   workqueue
+
 .. only::  subproject
 
    Indices

MAINTAINERS

@@ -13101,7 +13101,7 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq.git
 S:	Maintained
 F:	include/linux/workqueue.h
 F:	kernel/workqueue.c
-F:	Documentation/workqueue.txt
+F:	Documentation/core-api/workqueue.rst
 
 X-POWERS MULTIFUNCTION PMIC DEVICE DRIVERS
 M:	Chen-Yu Tsai <wens@csie.org>