Docs: add AMQP_SASL_METHOD_EXTERNAL documentation.
Docs: add AMQP_SASL_METHOD_EXTERNAL documentation.

Add docs to amqp_login* functions on how to correctly use
AMQP_SASL_METHOD_EXTERNAL to authenticate to the broker.

Fixes #349

Haster2004/rabbitmq-c,alanxz/rabbitmq-c,alanxz/rabbitmq-c,Haster2004/rabbitmq-c,Haster2004/rabbitmq-c,alanxz/rabbitmq-c