[doc] GetBestBlock() doesn't do nothing

This has tripped people up multiple times because it looks like
GetBestBlock is a const function returning the value of hashBlock.

src/validation.cpp

@@ -690,7 +690,8 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
         }
     }
 
-    // Bring the best block into scope
+    // This is const, but calls into the back end CoinsViews. The CCoinsViewDB at the bottom of the
+    // hierarchy brings the best block into scope. See CCoinsViewDB::GetBestBlock().
     m_view.GetBestBlock();
 
     // we have all inputs cached now, so switch back to dummy (to protect