doc: Add returning code conventions

Zephyr now uses codes from errno.h as the standard returning code for
all APIs. Since some code usages can be confusing or ambiguous, this
patch documents the conventions we are using to keep the returning code
usage consistent.

The document is based on the current mapping we have in device.h.

Change-Id: Ie6f429602592be8572631e34ae65435530a05b9f
Signed-off-by: Andre Guedes <andre.guedes@intel.com>