Improve documentation for scale_*_gradient

[paleolimbot]

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

• In scale_*_gradient(), it may be worth mentioning that passing na.value = NA is useful when used with the fill aesthetic (fill = NA results in no fill), but will not work for the colour aesthetic.

• In scale_*_gradient(), the behaviour of midpoint when a trans is applied should be documented (depends on resolution of midpoint is not transformed in scale_color_gradient2 #3198).

[lock]

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