Auto merge of #3856 - mikerite:clippy-dev-enchancement-1, r=phansch
Auto merge of #3856 - mikerite:clippy-dev-enchancement-1, r=phansch

Improve Clippy dev help

+ Print help if no subcommand is supplied
+ Make a short version of `update_lints` help for the subcommand listing

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust