Clarify what a restart means (#966)

* Clarify what a restart means

* Review feedback

* Fix review comments

Author: mlucool

docs/messaging.rst

@@ -1049,6 +1049,15 @@ multiple cases:
   IPythonQt client) to force a kernel restart to get a clean kernel without
   losing client-side state like history or inlined figures.
 
+Implementation recommendation for starting kernels: A restart should optimally
+preserve as many resources outside the kernel as possible (e.g. only restart the
+kernel and its subprocesses and not any parent processes). That is, ideally a
+restart should be "in-place". For local kernels, there is typically no parent
+process so a "hard" restart and an in-place restart are identical whereas for
+remote kernels this is not generally the same. As an example, if a remote kernel
+is run in a container, during an in-place restart the container may be kept
+running and a new kernel process within it would be started.
+
 The client sends a shutdown request to the kernel, and once it receives the
 reply message (which is otherwise empty), it can assume that the kernel has
 completed shutdown safely.  The request is sent on the ``control`` channel.