docs/core-api/mm: fix GFP combinations section name

Fix the mismatch between "Useful GFP flag combinations" section naming in
the DOC: section in include/linux/gfp.h and
Documentation/core-api/mm-api.rst.  This brings in the documentation,
and eliminates one warning:

    ./include/linux/gfp.h:1: warning: no structured comments found
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
[jc: tweaked changelog]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/core-api/mm-api.rst

@@ -35,7 +35,7 @@ users will want to use a plain ``GFP_KERNEL``.
    :doc: Reclaim modifiers
 
 .. kernel-doc:: include/linux/gfp.h
-   :doc: Common combinations
+   :doc: Useful GFP flag combinations
 
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