cmd/cleanup: add description about all to --prune

[hyuraku]

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add description about all to --prune=in cmd/cleanup. Because only the error message tells about all.

[MikeMcQuaid]

Suggested change

	description: "Remove all cache files older than specified <days>."\
	"if you want to remove the all, set 'all'"
	description: "Remove all cache files older than specified <days>. "\
	"If you want to remove everything, use `--prune=all`."

[hyuraku]

@MikeMcQuaid
Could you review this again?

[MikeMcQuaid]

Sorry, just noticed the missing space/capital here. Sorry again for the delay in rereview!

[MikeMcQuaid]

Suggested change

	description: "Remove all cache files older than specified <days>."\
	"if you want to remove everything, use `--prune=all`."
	description: "Remove all cache files older than specified <days>. "\
	"If you want to remove everything, use `--prune=all`."

[hyuraku]

Thank you for the review. I fix it right now.

[MikeMcQuaid]

Thanks again @hyuraku!