Explain the purpose of [[target.starknet-contract]] in context of the optimized test compilation architecture

[cptartur]

Which components does the task require to be changed? (think hard pls)

snforge

Description

When not using --no-optimization flag with snforge test, the [[target.starknet-contract]] is unnecessary for running tests as contracts will be compiled along them anyway.
However, having it defined with sierra = true is still required for --no-optimization run.

This should be properly explained in relevant documentation section.