Document other-extensions in user guide

[hvr]

I was surprised to see that the user-guide doesn't mention the other-extensions field at all.

[asr]

Meanwhile, could someone tell me the difference between the extensions and other-extensions fields?

[asr]

I found the answer. Using cabal-version: >= 1.10, cabal-install 1.20.0.2 generates the following warning:

Warning: For packages using 'cabal-version: >= 1.10' the 'extensions' field is
deprecated. The new 'default-extensions' field lists extensions that are used
in all modules in the component, while the 'other-extensions' field lists
extensions that are used in some modules, e.g. via the {-# LANGUAGE #-}
pragma.

[bicycle1885]

And the document about 'default-extensions' is also missing.

[hvr]

This has been partly addressed by 3f53ee2 but as @phadej points out it should provide more guidance about what effect using other-extensions has (e.g. for Cabal < 1.24 it results in Cabal aborting if the current compiler doesn't provide the extensions, for Cabal>=1.24 the solver uses that information)

[23Skidoo]

Fixed by #2818.

[massysett]

Thank you!

[Ericson2314]

https://www.haskell.org/cabal/users-guide/ seems really out of date---this is probably documented elsewhere but I've twice forgotten about this, been confused by the lack of documentation at the URL, and then checked the repo.

[hvr]

@Ericson2314 for future reference, bleeding edge docs are at http://cabal.readthedocs.io/en/latest/developing-packages.html#build-information