[SyntaxTreeDeserializer] Update comments for WeakLookupTable

Author: rintaro

Sources/SwiftSyntax/SwiftSyntax.swift

@@ -54,9 +54,6 @@ extension RawSyntax : Identifiable {}
 /// instance of the deserializer must be used for all subsequent
 /// deserializations.
 public final class SyntaxTreeDeserializer {
-  // FIXME: This lookup table just accumulates nodes, we should invalidate nodes
-  // that are no longer used at some point and remove them from the table
-
   /// Syntax nodes that have already been parsed and are able to be reused if
   /// they were omitted in an incremental syntax tree transfer
   private var nodeLookupTable: WeakLookupTable<RawSyntax> = .init()
@@ -65,7 +62,7 @@ public final class SyntaxTreeDeserializer {
   /// `nodeLookupTable`. Because `nodeLookupTable` only holds a weak reference 
   /// to the RawSyntax nodes, all retired `RawSyntax` nodes will be deallocated
   /// once we set a new tree. The weak references in `nodeLookupTable` will then
-  /// become `nil` but will also never be accessed again.
+  /// become `nil` and the slot will be reused to refer another node.
   private var nodeLookupTree: RawSyntax? = nil
 
   /// The IDs of the nodes that were reused as part of incremental syntax