lib: add extensive API documentation

lib/wp/base-endpoint.c

lib/wp/client.c

lib/wp/client.h

lib/wp/configuration.c

lib/wp/configuration.h

lib/wp/core.c

lib/wp/device.c

lib/wp/device.h

lib/wp/endpoint.c

lib/wp/endpoint.h

lib/wp/error.c

lib/wp/error.h

lib/wp/factory.c

lib/wp/factory.h

lib/wp/link.c

lib/wp/link.h

lib/wp/module.c

lib/wp/module.h

lib/wp/node.c

lib/wp/node.h