BUG: New natural_* landuse need custom sort_key

[nvkelso]

In #980 we added a few more kind values to differentiate from the landuse tags with the same values:

• natural_wood
• natural_forest
• natural_park

We also need to add those new kind values to the documentation, and they're getting default sort_rank values right now (so they z-stack below every other landuse incorrectly).

• Update layer docs
• Update sort_rank CSVs

The other "natural" features are here:

wetland,*,35
scrub,*,34
meadow,*,33
grass,*,32

So we need to add the 3 new ones, and offset the rest up (including ALL the rest, not just these 4):

wetland,*,38
scrub,*,37
meadow,*,36
grass,*,35
natural_wood,34
natural_forest,33
natural_park,32

• What map location are you having problems with? New York city's Prospect Park has some natural_wood features:
  • http://tile.dev.mapzen.com/mapzen/vector/v1/all/15/9650/12326.topojson

/cc @bcamper, thanks for finding the bug!

[nvkelso]

Verified.