C API: biscuit_builder_build says that "The builder will be freed automatically when the biscuit is returned" but that's false

[tudor-openai]

biscuit_builder_build takes the builder argument by reference (Option<&BiscuitBuilder>) so it can't possibly free it.

(Contrast with authorizer_builder_build which takes Option<Box<AuthorizerBuilder>>, so it frees on exit by losing the reference.

IMO we shouldn't try to change the behavior now, but just document the difference; code that expects the current behavior would do a double free if we change it.