Document NGINX_USE_LOCAL_RESOLVER in the docs

[thresheek]

Is your feature request related to a problem? Please describe

NGINX_USE_LOCAL_RESOLVER needs to be documented in the docs, see #673 for more details

[agebhar1]

The variable is NGINX_ENTRYPOINT_LOCAL_RESOLVERS which must be non empty to provide NGINX_LOCAL_RESOLVERS with nameserver from /etc/resolv.conf.