docs/core-api: Add string helpers API to the list

Some times string helpers are needed, but there is nothing about them
in the generated documentation.

Fill the gap by adding a reference to string_helpers.c exported functions.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/core-api/kernel-api.rst

@@ -33,6 +33,9 @@ String Conversions
 .. kernel-doc:: lib/kstrtox.c
    :export:
 
+.. kernel-doc:: lib/string_helpers.c
+   :export:
+
 String Manipulation
 -------------------