Document how embedders release and re-enter the runtime #84

Embedders that call main more than once (wasm callMain, plugin hosts) keep the old top of stack, since SetTopOfStack only widens it unless the thread is released with hx::SetTopOfStack(0,false), and the generated main always returns 0. The release call and its limits (statics are not re-initialised) should be documented, with a small helper if needed.