Documentation: core-api: Add error pointer functions to kernel-api

Bring the error pointer functions (e.g. ERR_PTR(), PTR_ERR()) into
the docs build so that they can be cross-referenced elsewhere.

List them as kernel library functions in the kernel-api document.
Nowhere else seems to fit, and they need to go *somewhere*.

Signed-off-by: James Seo <james@equiv.tech>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230509175543.2065835-4-james@equiv.tech