docs: joinTableAttributes in belongsToMany mixin

[cincodenada]

This address sequelize/website#97 partially at least - I'm not sure where else it should be documented.

[cincodenada]

I'm still not quite sure why this doesn't work - I see that we're manually adding an entry to the include array in belongs-to-many.js

I'm guessing this is so that we have the join model attributes by default, and because we're not controlling the join, it would be awkward to try to add it. A naive attempt at suppressing the join table would look something like:

Foo.getBars({
    include: [{
      model: Bar,
      through: { attributes: [] }
    }]
})

but that would actually attempt to join the returned Bars in turn to Bar, which won't work (or at least won't have the intended effect, and I'm not sure it's even possible for the end-user to replicate the join that's generated reasonably.

I don't know the best way to explain that succinctly - I've done my best here.

[github-actions]

🎉 This PR is included in version 7.0.0-alpha.6 🎉

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• npm package (@release-v7 dist-tag)
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[yisibl]

Thank you very much for adding the documentation.