Document that singleton validators, mappers and event handlers are built from the root provider

[mbcrawfo]

Summary

Validators, mappers, processors and event handlers are singletons. They were built from the scope of the first request that needed them, so injecting a scoped service into their constructors captured that request's instance silently. FastEndpoints/FastEndpoints#1179 builds them from the root service provider instead.

• Validator Dependencies: say validators are built from the root provider. Injecting a scoped dependency throws when DI scope validation is on (Development by default), but in Production it silently becomes a shared singleton, so the error can't be relied on alone.
• Entity Mapper, Pre/Post Processor and Event Handler Dependencies: note the same root provider construction.

Validation

• Changed file passes Prettier
• Production documentation build passes

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