docs: add disclaimer about non-Hamiltonian flows (JOSS reviewer @giovannifereoli, round 2)

[ocots]

Context

This PR addresses point 2 of the second-round review by @giovannifereoli for the JOSS submission (openjournals/joss-reviews#10630).

The reviewer asked for a clear disclaimer about the current limitation of indirect methods to Hamiltonian flows, and the planned future support for non-Hamiltonian flows.

Changes

joss/paper.md

Extended the Indirect methods bullet in the Software Design section with a closing sentence:

Currently, indirect methods are limited to Hamiltonian flows, where the feedback control law depends on both state and costate; support for non-Hamiltonian flows (open-loop or state-feedback control laws) is under active development.

docs/src/manual-flow-ocp.md

Added a note admonition at the very top of the flow-from-OCP tutorial, before any code example:

Current limitation: Currently, from an optimal control problem, only Hamiltonian flows can be constructed, i.e. the control law must be provided in feedback form depending on both state and costate: u(x, p). An active refactoring is under way to extend support to non-Hamiltonian flows, in particular open-loop control laws u(t) and state feedback control laws u(x).

This placement is the most visible for users actually working with the Flow function from an OCP.