net: coap: Use MQTT style wildcard in path description:

In 'struct coap_resource' path description:
- the plus symbol represents a single-level wild card in the path;
- the hash symbol represents the multi-level wild card in the path.

This change keeps compatibility with RFC 7252 but allows handling
multiple requests in single function.

Signed-off-by: Eug Krashtan <eug.krashtan@gmail.com>