Auto merge of #2788 - Manishearth:improve-help, r=alexcrichton
Auto merge of #2788 - Manishearth:improve-help, r=alexcrichton

Clarify where full list of subcommands can be found

None

sbeckeriv/cargo,mark-buer/cargo,sbeckeriv/cargo,mark-buer/cargo,tshepang/cargo,tshepang/cargo,tsurai/cargo,eminence/cargo,durka/cargo,rustyhorde/cargo,vojtechkral/cargo,nipunn1313/cargo,eminence/cargo,nipunn1313/cargo,durka/cargo,mark-buer/cargo,achanda/cargo,tsurai/cargo,rustyhorde/cargo,achanda/cargo,AndiDog/cargo,vojtechkral/cargo,mark-buer/cargo,AndiDog/cargo,brson/cargo,durka/cargo,rust-lang/cargo,sbeckeriv/cargo,brson/cargo,tshepang/cargo,eminence/cargo,rust-lang/cargo,brson/cargo,nipunn1313/cargo,vojtechkral/cargo,rust-lang/cargo,rustyhorde/cargo,achanda/cargo,tshepang/cargo,AndiDog/cargo,AndiDog/cargo,durka/cargo,tsurai/cargo,rust-lang/cargo,brson/cargo,eminence/cargo,nipunn1313/cargo,achanda/cargo,tsurai/cargo,vojtechkral/cargo,tsurai/cargo,AndiDog/cargo,mark-buer/cargo,brson/cargo,sbeckeriv/cargo,rustyhorde/cargo