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>