fix discrete_scale documentation

[alistaire47]

Updates discrete_scale documentation to match functionality and continuous_scale to make it clear the labels parameter can accept a formatter function as shown in the last example of scale_x_discrete (fixes #2052).

No code changes; just literally coped and pasted the documentation of the labels parameter from continuous_scale, called devtools::document(), and updated NEWS.md per CONTRIBUTING.md (delete that if it's too minor for news!).

[hadley]

Can you please use @inheritParams instead?

[alistaire47]

I was putting this together but realized continuous_scale currently actually inherits parameters from discrete_scale, even though it has all its parameters defined anyway. They each have a couple unique parameters, so do you have a preference for which inherits from which?

[hadley]

Not really. My general convention has been when there's a class of similar function, inherit documentation from the first in the alphabet, but it's not a big deal.

[alistaire47]

The newest commit merges shared parameters into continuous_scale, using whichever was more comprehensive, and now discrete_scale inherits from that. I left distinct parameters and the separate explanation of na.value that relates to na.translate.

I also realized the explanations of the scale_name and name parameters is repetitive and unclear, but I don't know enough about the difference and how they behave to alter anything.

[hadley]

Thanks!

Do you want to have a stab at clearing up name vs scale_name? scale_name is the name of the scale function like gradient2, brewer etc. name is the name of the scale applied to a specific plot. The user never sees scale_name so it's less important to document well (but it would be nice if it was less confusing)

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