Document HANDLE requirements for terminating processes (#16)

* Document HANDLE requirements for terminating processes

* Added link to CLI docs

Minor formatting fixes

---------

Co-authored-by: Benjamin Höglinger-Stelzer <nefarius@dhmx.at>

Author: fredemmott

docs/projects/Vicius/Terminate-Process-before-Update.md

@@ -1,3 +1,26 @@
 # Process Termination on Update
 
-TBD
+If the user chooses to install an update, the updater will terminate the specified process if [`--terminate-process-before-update <handle>`](Command-Line-Arguments.md#-terminate-process-before-update-handle) is passed on the command line; this is intended for when the updater is launched by the product that is being updated.
+
+This handle:
+
+- **MUST** be a Win32 `HANDLE`
+- **MUST** have `PROCESS_TERMINATE | PROCESS_QUERY_LIMITED_INFORMATION` permissions
+- **MUST** be inheritable, and owned by the updaters' parent process
+- **MUST NOT** be a pseudo-handle, e.g. you can not directly use the return value of `GetCurrentProcess()`
+
+For example, you can create the handle with:
+
+```c++
+HANDLE handle {};
+DuplicateHandle(
+    GetCurrentProcess(),
+    GetCurrentProcess(),
+    GetCurrentProcess(),
+    &handle,
+    PROCESS_TERMINATE | PROCESS_QUERY_LIMITED_INFORMATION,
+    /* inheritable = */ true,
+    /* flags = */ 0);
+```
+
+You must also specify that handles are inherited when calling `CreateProcess()` to launch the updater.