Document the disabling of flags

[philnate]

Hi,

given that kingpin has a special way to handle boolean flags, the documentation should be updated to showcase howto disable flags. I.e. at least to me it wasn't intuitive that I had to set the flag --es.aliases to --no-es.aliases to disable it. The same type of request had been raised by a couple of other exporter projects as well, see prometheus/consul_exporter#177 for a reference.