Update Nimbus docs on FeatureHolder.value exceptions and signature

[jonalmeida]

The docs for a generated FeatureHolder in Nimbus explains:

/**
 * Get the JSON configuration from the Nimbus SDK and transform it into a configuration object as specified
 * in the feature manifest. This is done each call of the method, so the method should be called once, and the
 * result used for the configuration of the feature.
 *
 * The unused parameter `Context` was relevant for `Text` and `Image` typed properties, but is no longer necessary.
 * It will be removed in later releases.
 *
 * @returns T
 * @throws NimbusFeatureException thrown before the Nimbus object has been constructed or `FxNimbus.initialize` has not been set.
 * This can be resolved by setting `FxNimbus.initialize`, and after that by passing in a `Context` object.
 */
@Suppress("UNUSED_PARAMETER")
fun value(context: Context? = null): T

This is no longer accurate to say that this method throws any longer.

The parameter context is also un-used and should be removed to avoid confusion.

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