docs: add doc-comments on GC-related methods

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

gc/gc.go

@@ -59,6 +59,8 @@ type Stats interface {
 //
 // We can probably use this to inform a design for incremental GC by injecting
 // callbacks to the set modification algorithms.
+//
+// https://en.wikipedia.org/wiki/Tracing_garbage_collection#Tri-color_marking
 func Tricolor(roots []Node, refs func(ref Node) ([]Node, error)) (map[Node]struct{}, error) {
 	var (
 		grays     []Node                // maintain a gray "stack"