Document `//export` requirement

[aykevl]

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The page here says the following:

// This function is imported from JavaScript, as it doesn't define a body.
// You should define a function named 'main.add' in the WebAssembly 'env'
// module from JavaScript.
func add(x, y int) int

That's outdated, and needs to be updated.

[alvarolm]

@aykevl thanks for pointing this out ! really interested in the correct procedure, it should be done using syscall/js ?