Mention estimation caveat in docs

[MujkicA]

This section in the code describes a caveat with estimation and output variables that we should mention in the docs somewhere:

https://github.com/FuelLabs/fuels-rs/blob/80fe445b0f1f6523e6fca83e88052bf608e27324/packages/fuels-core/src/types/transaction_builders.rs#L557