Document trait bounds in great detail

So we know what the hell we were doing if we ever have to revisit this

geo/src/algorithm/bool_ops/mod.rs

@@ -192,9 +192,15 @@ pub trait UnaryUnion {
 // "reduce" operations on parent nodes combine these output MultiPolygons from leaf operations by recursion
 fn bottom_up_fold_reduce<T, S, I, F, R>(ops: &Ops<I, F, R>, parent: &ParentNode<T>) -> S
 where
+    // This operation only requires two types: output (S) and input (T)
     T: RTreeObject,
+    // Because this is a fold operation, we need to initialise a "container" to which we'll be adding using union.
+    // The output of a union op is a MultiPolygon.
     I: Fn() -> S,
+    // The meat of the fold op is unioning input borrowed leaf Polygons into an output MultiPolygon.
     F: Fn(S, &T) -> S,
+    // Parent nodes require us to process their child nodes to produce a MultiPolygon. We do this recursively.
+    // This is a reduce op so there's no need for an init value and the two inputs must have the same type: MultiPolygon
     R: Fn(S, S) -> S,
 {
     parent