[ntuple] Restrict the scope of descriptor guard in ntuple_storage.

Also update the outdated comment about GetNEntries()

Author: silverweed

tree/ntuple/inc/ROOT/RPageStorage.hxx

@@ -761,7 +761,7 @@ public:
    /// The underlying `std::shared_mutex`, however, is neither read nor write recursive:
    /// within one thread, only one lock (shared or exclusive) must be acquired at the same time. This requires special
    /// care in sections protected by `GetSharedDescriptorGuard()` and `GetExclDescriptorGuard()` especially to avoid
-   /// that the locks are acquired indirectly (e.g. by a call to `GetNEntries()`). As a general guideline, no other
+   /// that the locks are acquired indirectly. As a general guideline, no other
    /// method of the page source should be called (directly or indirectly) in a guarded section.
    const RSharedDescriptorGuard GetSharedDescriptorGuard() const
    {

tree/ntuple/test/ntuple_storage.cxx

@@ -1181,8 +1181,10 @@ TEST(RPageSourceFile, OpenDifferentAnchor)
    auto source = std::make_unique<RPageSourceFile>("ntpl1", fileGuard.GetPath(), RNTupleReadOptions());
    source->Attach();
    EXPECT_EQ(source->GetNEntries(), 100);
-   auto desc = source->GetSharedDescriptorGuard();
-   EXPECT_NE(desc->FindFieldId("f"), ROOT::kInvalidDescriptorId);
+   {
+      auto desc = source->GetSharedDescriptorGuard();
+      EXPECT_NE(desc->FindFieldId("f"), ROOT::kInvalidDescriptorId);
+   }
 
    auto anchor2 = file->Get<ROOT::RNTuple>("ntpl2");
    ASSERT_NE(anchor2, nullptr);