docs/core-api: move *{str,mem}dup* to "String Manipulation"

The string and memory duplication routines fit better to the "String
Manipulation" section than to "The SLAB Cache".

Link: http://lkml.kernel.org/r/1532626360-16650-5-git-send-email-rppt@linux.vnet.ibm.com
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/core-api/kernel-api.rst

@@ -39,6 +39,10 @@ String Manipulation
 .. kernel-doc:: lib/string.c
    :export:
 
+.. kernel-doc:: mm/util.c
+   :functions: kstrdup kstrdup_const kstrndup kmemdup kmemdup_nul memdup_user
+               vmemdup_user strndup_user memdup_user_nul
+
 Basic Kernel Library Functions
 ==============================
 
@@ -168,7 +172,7 @@ The Slab Cache
    :export:
 
 .. kernel-doc:: mm/util.c
-   :export:
+   :functions: kfree_const kvmalloc_node kvfree get_user_pages_fast
 
 User Space Memory Access
 ------------------------