[feature request] Make `devdocs-install` exclude already installed docs.

[astoff]

If you call devdocs-install on an already installed document, it will be reinstalled / updated, which is an occasionally useful feature.

I understand why in a full-blown package manager you would like to have separate installation and update commands. But this is not a full-blown package manager... Can you explain why you want this feature?

[stardiviner]

Ok, I see, thanks for your explaination.