docs(plugins): note side-effect when using await in `fastify.regist…

…er()` (#4846)

* Add documentation for side-effect when using await in fastify.register()

* Apply suggestions from code review

Co-authored-by: Frazer Smith <frazer.dev@outlook.com>

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Co-authored-by: Uzlopak <aras.abbasi@googlemail.com>
Co-authored-by: Frazer Smith <frazer.dev@outlook.com>

docs/Reference/Plugins.md

@@ -145,6 +145,10 @@ await fastify.ready()
 
 await fastify.listen({ port: 3000 })
 ```
+*Note: Using `await` when registering a plugin loads the plugin
+and the underlying dependency tree, "finalizing" the encapsulation process.
+Any mutations to the plugin after it and its dependencies have been
+loaded will not be reflected in the parent instance.*
 
 #### ESM support
 <a id="esm-support"></a>