Update documentation regarding derivability of Generic1

[spl]

The  documentation for Generic1 says:

Representable types of kind * -> * (not yet derivable).

I'm not sure if this feature is planned or not, but it would certainly be nice to have. Without it, I suspect the Generic1 representation will not see much use, and it doesn't make much sense to write generic functions for it.

[dreixel]

It is derivable in 7.6. I'll update the haddock, thanks for noticing.

[dreixel]

Fixed by ec8d1495f193257d3b42ac0e67fbe0aa59101181 in the base library repo.

[spl]

Excellent! I now see it in the 7.6.1 release notes. (I think it should have been in the highlights, myself.)

Now, I finally have a reason to install 7.6.1. ;)

[igloo]

Merged as 066fb9edecb3f293dbab36b87317100722a3c57c