Add some comments in check that required some reverse engineering

While reading check I had to figure some things out that were not
trivial, and I saved them with comments.

toolchain/check/check.cpp

@@ -44,6 +44,8 @@ struct UnitInfo {
         : package_id(package_id), node_id(node_id) {}
 
     // The identifier of the imported package.
+    //
+    // The current package uses the `IdentifierId::Invalid` id.
     IdentifierId package_id;
     // The first `import` declaration in the file, which declared the package's
     // identifier (even if the import failed). Used for associating diagnostics
@@ -76,7 +78,8 @@ struct UnitInfo {
   // name lookup.
   llvm::SmallVector<PackageImports> package_imports;
 
-  // A map of the package names to the outgoing imports above.
+  // A map of the package names to the outgoing imports above. The
+  // `IdentifierId::Invalid` id refers to the current package.
   Map<IdentifierId, int32_t> package_imports_map;
 
   // The remaining number of imports which must be checked before this unit can

toolchain/check/context.h

@@ -691,6 +691,8 @@ class Context {
   // Per-import constant values. These refer to the main IR and mainly serve as
   // a lookup table for quick access.
   //
+  // Indexed by the integer values of the `SemIR::ImportIRId`s in import_irs().
+  //
   // Inline 0 elements because it's expected to require heap allocation.
   llvm::SmallVector<SemIR::ConstantValueStore, 0> import_ir_constant_values_;