Improve help for tmctl config set

[jmcx]

While working on docs about updating tmctl, I noticed we don't describe the possible options we can set. On the other hand, it does show them for tmctl config get (as long as you don't add -h).

~/TriggerMesh % tmctl config set -h
Write config value

Usage:
  tmctl config set <key> <value> [flags]

Flags:
  -h, --help   help for set

Global Flags:
      --broker string    Optional broker name.
      --version string   TriggerMesh components version.