Add a note regarding importsNotUsedAsValues to docs

[freshgum-bubbles]

The TypeScript compiler can be told to remove imports from which
values aren't used. Ultimately, this is harmful to consumers of TypeDI,
as any @Service calls need to run to be read by the container.

Therefore, a warning should be added to the documentation.

Ref: typestack/typedi#778

[github-actions]

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