Update Slab/PerimeterInsulationDepth documentation

[shorowit]

HPXML defines Slab/PerimeterInsulationDepth as "Depth from grade to bottom of vertical slab perimeter insulation". That makes sense for a slab-on-grade foundation, which is where perimeter insulation is most commonly used, but it should be changed to "Depth from top of slab to bottom of vertical slab perimeter insulation" to more generally accommodate slabs below grade too.