Don't use union types in context and API definitions

[kriswest]

resolves #1105

Adds content to the Context and API overviews about not using unions types (particularly of primitive types) as they are not easily implementable in other languages.

Preview deeplinks:

• https://deploy-preview-1200--fdc3.netlify.app/docs/next/context/spec#field-type-conventions
• https://deploy-preview-1200--fdc3.netlify.app/docs/next/api/spec#implementation-language

[netlify[bot]]

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