#4475: added docs on file backend method, pr feedback

exp/lighthorizon/index/backend/file.go

@@ -20,6 +20,8 @@ type FileBackend struct {
  parallel uint32
 }
 
+// NewFileBackend connects to indices stored at `dir`, creating the directory if one doesn't 
+// exist, and uses `parallel` to control how many workers to use when flushing to disk.
 func NewFileBackend(dir string, parallel uint32) (*FileBackend, error) {
  if parallel <= 0 {
  parallel = 1