docs/core-api: Add integer power functions to the list

Some times integer power functions, such as int_sqrt(), are needed, but
there is nothing about them in the generated documentation.

Fill the gap by adding a reference to the corresponding 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>