Documentation: move watch_queue to core-api

Move watch_queue documentation to the core-api index and
subdirectory.

Fixes: c73be61ced ("pipe: Add general notification queue support")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/core-api/index.rst

@@ -18,6 +18,7 @@ it.
 
    kernel-api
    workqueue
+   watch_queue
    printk-basics
    printk-formats
    symbol-namespaces

Documentation/index.rst

@@ -168,7 +168,6 @@ to ReStructured Text format, or are simply too old.
 
    tools/index
    staging/index
-   watch_queue
 
 
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