Add module-level documentation for restarting and multi-machine event managers

**Is your feature request related to a problem? Please describe.**

While going through the event manager implementations, I found it hard
to understand the intent and correct usage of `restarting.rs` and
`multi_machine.rs` because they don’t have any module level documentation or
a simple introduction.

Most other event managers such as tcp,event_hooks,llmp include a
short comment at the top explaining what they do and when to
use them, but these two don’t, which makes it difficult
for new contributors to quickly grasp their purpose.

**Describe the solution you'd like**

Add short module level rustdoc comments to `restarting.rs` and
`multi_machine.rs` which explain what each event manager does and
when it should be used.

For example, a one–two lines introduction at the top of each file
(similar to the other event managers) describing the purpose of the
restarting event manager and how the multi-machine setup works at a high
level. This would make them much easier to understand and consistent with
the rest of the codebase.

**Describe alternatives you've considered**

Reading through the source code and related examples is possible, but without
some small documentation explaining a bit about these event managers,
it will require a significant amount of time and prior architectural
knowledge to figure it out. Adding these short and conceptual docs
seems like the simplest and most effective improvement.