#2719 introduces multi-stage install, via a single positional argument:

• linkerd install config --flags
• linkerd install control-plane --flags

The help output does not provide any context on which install flags are supported for each stage. We should make that explicit in the code and help output. This could be accomplished by new top-level cli commands, or via cobra subcommands on install.

Motivated by: #2719 (comment)