tooling: --help option for crystal docs

[scriptnull]

I am very new to crystal and have been playing around with the tools.

Almost all the crystal commands have a --help option to help the user to understand its purpose better. But the crystal docs command does not seem to have one.

I think it would be better to have some help for it like below.

$ crystal docs
Usage: crystal docs [Options] [file]

Options:
    -h, --help      Show this message

In this way, it would be easy for adding new options and documenting them in future. (Right now crystal docs has no options, I suppose)

[straight-shoota]

#4937 includes a --help option.
But we should keep this issue open until it gets merged.

[asterite]

#4937 got merged so this is fixed now