docs: Add missing docstrings, adjust wording to match standard style

This patch adds a missing docstrings for exported identifiers, and
adjust some of the existing ones to match the standard style.

In some cases, the identifiers were un-exported after noticing they had
no external users.

Besides improving documentation, it also reduces the linter noise
significantly.

internal/domaininfo/domaininfo.go

@@ -13,6 +13,7 @@ import (
 // Command to generate domaininfo.pb.go.
 //go:generate protoc --go_out=. domaininfo.proto
 
+// DB represents the persistent domain information database.
 type DB struct {
 	// Persistent store with the list of domains we know.
 	store *protoio.Store
@@ -23,6 +24,8 @@ type DB struct {
 	ev *trace.EventLog
 }
 
+// New opens a domain information database on the given dir, creating it if
+// necessary. The returned database will not be loaded.
 func New(dir string) (*DB, error) {
 	st, err := protoio.NewStore(dir)
 	if err != nil {