n-api documentation does not list minimum version of types

[josephg]

Each type in n-api is only available from some particular n-api version. But the documentation doesn't list the minimum version from which each API is available.

For example, napi_threadsafe_function is only available from NAPI_VERSION 4 onwards. And thus, it can't be used if you're hoping to support older versions of nodejs. But its impossible to know that from the documentation alone - I had to dig into the napi header files to figure out what APIs I can use.

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Edit: It looks like the N-API version is listed for functions, just not for types. So this is a pretty minor issue.

[benjamingr]

@josephg sounds reasonable to list it on the types and not just the functions to me. Docs PR welcome :]