Improve documentation for scale_*_brewer()

[paleolimbot]

There are several opportunities to improve the documentation for scale_*_brewer(). This is a documentation issue intended for the tidyverse dev day (#3267).

• It's worth mentioning that the "Palettes" section in scale_*_brewer() refers to the palette argument (and mention that there is a "Palettes" section in the argument documentation for palette). I know this seems obvious but I've answered this question a few times in tutorials.

• I'm not sure we need to demonstrate how to change scale label in the examples section for scale_*_brewer(), or the appearance of various scales on a dark background.

[lock]

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