Adding docs for SetEnvPrefix and cleaning up docs.
Adding docs for SetEnvPrefix and cleaning up docs.

spf13/viper,mirrr/viper,akutz/viper,jrossi/viper,DLag/viper,zj8487/viper,vasu1124/viper,dvln/viper,streamrail/viper,ishidawataru/viper,samt42/viper,istio/viper,niccaluim/viper,henrylee2cn/viper,sankark/viper,mguindin/viper