fix(cmd_help): make help support docgen argument

[ee7]

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Issue

#170

Description

Before this PR, chalk help docgen didn't behave as I'd expect:

$ chalk help docgen
 Searching Documentation For Term: Docgen
 No Matches Found In Command-Line Flag Documentation
 No Matches In Configuration Variables
 No Matches In Found Metadata Key Docs
 No Matches In Other Documents.

Make it work, producing the same output as chalk docgen --help:

$ chalk help docgen
 docgen command
 Generate technical documentation

 ┌┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┬┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┐
 ┊ Aliases                              ┊ None                ┊
 ┊ Arguments                            ┊ None                ┊
 └┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┴┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┘

 Description
Internal function to generate technical documentation in
markdown format.

Fixes: #170

Testing

Run chalk help docgen.

[miki725]

LGTM although docgen was sort of internal command so not sure if @viega omitted it from help system on purpose

[ee7]

docgen is currently listed in chalk help commands, so I think chalk help docgen should work given that chalk docgen --help works.

As a follow-up, we could consider not exposing chalk docgen in user-facing help messages if we think that regular chalk users don't need to know about it. There might be some complexity there if we want to mark docgen in the spec as an internal command.

I think this PR is at least an improvement on the status quo, so I'll merge it. If we want docgen to be more hidden, please open an issue for that. We could also consider e.g. changing the description from "Generate technical documentation" to something like "Internal command to generate chalk documentation".