Class diagram

All of the Hook* classes are somewhat confusing to me, and I always have to rediscover how they relate. So I created an ASCII diagram of it. I think it's worth shoving in a comment somewhere, or in some "architecture" document. I'll use this issue as a placeholder for that.

                                                 ┌──────────┐
                                                 │ HookSpec │
                                                 └──────────┘
                                                       ▲       plugin1    ┌────────┐
                                                       │  ┌──────────────►│HookImpl│
                                                       │  │               └────────┘
                                       foo_hook1 ┌─────┴──┴─┐
                                      ┌──────────┤HookCaller├───►...
                                      │          └────────┬─┘
                                      │                   │               ┌────────┐
                                      │                   └──────────────►│HookImpl│
                                      │                        plugin2    └────────┘
    ┌─────────────┐  pm.hook  ┌───────┴─┐
    │PluginManager├──────────►│HookRelay├───►...
    └─────────────┘           └───────┬─┘
                                      │                        plugin2    ┌────────┐
                                      │                  ┌───────────────►│HookImpl│
                                      │                  │                └────────┘
                                      │          ┌───────┴──┐
                                      └─────────►│HookCaller├───►...
                                       foo_hook2 └─────┬─┬──┘
                                                       │ │                ┌────────┐
                                                       │ └───────────────►│HookImpl│
                                                       ▼       plugin3    └────────┘
                                                 ┌──────────┐
                                                 │ HookSpec │
                                                 └──────────┘