net: lwm2m: update function prototypes and descriptions

- Several of the functions use "path" as the parameter name for the
  string-based LwM2M path.  Let's clarify by using "pathstr".
- Recent updates to the LwM2M engine now support resource instances
  when parsing the LwM2M path.  Let's update descriptions accordingly.

Signed-off-by: Michael Scott <mike@foundries.io>