[snapshot] Improve snapshot docs and error printing

- Minor improvements to the documentation for snapshotting.
- Add newlines to printed errors where necessary.

Change-Id: I822e7e850adb67eae73b51c23cf34e40ba3106f0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2144954Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67111}

include/v8.h

@@ -9800,6 +9800,11 @@ class V8_EXPORT V8 {
 
 /**
  * Helper class to create a snapshot data blob.
+ *
+ * The Isolate used by a SnapshotCreator is owned by it, and will be entered
+ * and exited by the constructor and destructor, respectively; The destructor
+ * will also destroy the Isolate. Experimental language features, including
+ * those available by default, are not available while creating a snapshot.
  */
 class V8_EXPORT SnapshotCreator {
  public:
@@ -9828,6 +9833,10 @@ class V8_EXPORT SnapshotCreator {
   SnapshotCreator(const intptr_t* external_references = nullptr,
                   StartupData* existing_blob = nullptr);
 
+  /**
+   * Destroy the snapshot creator, and exit and dispose of the Isolate
+   * associated with it.
+   */
   ~SnapshotCreator();
 
   /**

src/snapshot/serializer-common.cc

@@ -76,7 +76,8 @@ ExternalReferenceEncoder::Value ExternalReferenceEncoder::Encode(
   if (maybe_index.IsNothing()) {
     void* addr = reinterpret_cast<void*>(address);
     v8::base::OS::PrintError("Unknown external reference %p.\n", addr);
-    v8::base::OS::PrintError("%s", ExternalReferenceTable::ResolveSymbol(addr));
+    v8::base::OS::PrintError("%s\n",
+                             ExternalReferenceTable::ResolveSymbol(addr));
     v8::base::OS::Abort();
   }
   Value result(maybe_index.FromJust());

src/snapshot/startup-serializer.cc

@@ -193,6 +193,7 @@ void SerializedHandleChecker::VisitRootPointers(Root root,
     PrintF("%s handle not serialized: ",
            root == Root::kGlobalHandles ? "global" : "eternal");
     (*p).Print();
+    PrintF("\n");
     ok_ = false;
   }
 }