Document guidance on hiding certain POI labels at lower zooms

[claysmalley]

I think implementing text rendering at a higher zoom level than icons is a nice approach for a wide variety of icons, including here. Since it'll only become more important as more icons are added, it might make sense to decide on a style-wide guidance for when to do and not do it, rather than continuing on an ad-hoc basis (though I don't think that resolving such a discussion should block implementing it here)

Originally posted by @wmisener in #999 (comment)

[wmisener]

For context, I think text is currently first rendered at a higher zoom than the symbol is for bus stops (icon appears at z15, label appears at z17) and gas stations (icon appears at z15, label appears at z16). I wonder if it makes sense for this to be extended to other "minor" POIs (cafes, bars, etc, for which text and icon currently appears at z16), or even more major ones (schools, places of worship, etc, which currently appear at z15).

Right now it's not too much of an issue, as icons are still pretty sparse. But as rendered features become more densely distributed, it might make sense to render some labels later for more features to avoid inconsistencies. It also provides another way to emphasize certain features by rendering their labels relatively early.