sys: slist, dlist: Document APIs as not threadsafe

These headers provide an efficient, inline implementations of single-
and double- linked lists, and thus not threadsafe. They are intended
to be used as internal kernel APIs (and currently for example not
documented at https://www.zephyrproject.org/doc/

). However, to avoid
issues when doing kernel programming (e.g. #4350), it makes sense
to explicitly, even verbosely, document these functions as not
threadsafe.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>