Add some shadow copy info (#35201)

Author: HaoK

src/Servers/IIS/AspNetCoreModuleV2/AspNetCore/applicationinfo.cpp

@@ -251,6 +251,26 @@ APPLICATION_INFO::ShutDownApplication(const bool fServerInitiated)
     }
 }
 
+/* Overview of shadow copy logic when enabled. See https://github.com/dotnet/aspnetcore/pull/28357 for more context
+
+ * On first request, ANCM goes through its startup sequence, starting dotnet and sending the request into managed code. During this sequence, 
+ * ANCM will copy the contents of the app directory to another directory which is user specified. The path to this directory can be absolute or relative.
+
+ * Logs and log files will be written to the app directory rather than the shadow copy directory. app_offline will also only be watched in the app directory. 
+ * The current directory will be set to the app directory as well as the AppContext.BaseDirectory.
+
+ * On publish of new content to the app directory, ANCM will start debouncing file change notifications for dlls, waiting for a steady state. 
+ * This is done by resetting a timer each time a dll is changed, eventually triggering the timer once there are no dll changes. Afterwards, shutdown is started, 
+ * causing the process to recycle.
+
+ * Subfolders are created under the user specified shadowCopyDirectory, where the highest int value directory name will be used each time. 
+ * It will start at subdirectory with name '0' and increment from there. On shutdown, because dlls are still locked by the running process, 
+ * we need to copy dlls to a different directory than what is currently running in the app. So in the case where the directory name is '0', 
+ * we will create a directory name '1' and write the contents there. Then on app start, it will pick the directory name '1' as it's the highest value.
+
+ * Other directories in the shadow copy directory will be cleaned up as well. Following the example, after '1' has been selected as the directory to use, 
+ * we will start a thread that deletes all other folders in that directory.
+ */
 std::filesystem::path
 APPLICATION_INFO::HandleShadowCopy(const ShimOptions& options, IHttpContext& pHttpContext)
 {