Document more Git.execute kill_after_timeout limitations

See discussion in gitpython-developers#1756.

Author: EliahKagan

git/cmd.py

@@ -879,11 +879,19 @@ def execute(
             Specifies a timeout in seconds for the git command, after which the process
             should be killed. This will have no effect if `as_process` is set to True.
             It is set to None by default and will let the process run until the timeout
-            is explicitly specified. This feature is not supported on Windows. It's also
-            worth noting that `kill_after_timeout` uses SIGKILL, which can have negative
-            side effects on a repository. For example, stale locks in case of ``git gc``
-            could render the repository incapable of accepting changes until the lock is
-            manually removed.
+            is explicitly specified. Uses of this feature should be carefully
+            considered, due to the following limitations:
+
+            1. This feature is not supported at all on Windows.
+            2. Effectiveness may vary by operating system. ``ps --ppid`` is used to
+               enumerate child processes, which is available on most GNU/Linux systems
+               but not most others.
+            3. Deeper descendants do not receive signals, though they may sometimes
+               terminate as a consequence of their parent processes being killed.
+            4. `kill_after_timeout` uses ``SIGKILL``, which can have negative side
+               effects on a repository. For example, stale locks in case of ``git gc``
+               could render the repository incapable of accepting changes until the lock
+               is manually removed.
 
         :param with_stdout:
             If True, default True, we open stdout on the created process.