add explanatory comments

Author: tayloraswift

Sources/UnidocDB/Packages/PackageDatabase.Editions.swift

@@ -88,9 +88,11 @@ extension PackageDatabase.Editions
         package:Int32,
         with session:Mongo.Session) async throws -> Int32?
     {
+        //  We use the SHA-1 hash as “proof” that the edition has at least one symbol graph.
+        //  Therefore, merely registering tags does not update hashes.
         let placement:Placement = try await self.register(package: package,
             refname: tag.name,
-            sha1: tag.hash,
+            sha1: nil,
             with: session)
 
         return placement.new ? placement.coordinate : nil

Sources/UnidocDB/Packages/PackageDatabase.swift

@@ -96,6 +96,9 @@ extension PackageDatabase
         else if case .swift = docs.metadata.package,
             let tagname:String = docs.metadata.commit?.refname
         {
+            /// Standard library symbol graphs don’t come with hashes, so we can’t efficiently
+            /// “prove” that a particular edition has at least one symbol graph. But we don’t
+            /// need to query that in the first place.
             let placement:Editions.Placement = try await self.editions.register(
                 package: placement.coordinate,
                 refname: tagname,