Please, document the removal of the C API

[edgar-bonet]

The documentation still advertises a C API. However, on Release-2025.2, this API is no more: it has been replaced by a C++ API.

It should also be mentioned that this new API is only compatible with C++20 or higher (as rp_acq_axi.h uses std::span). This is
relevant because the provided C++ compiler defaults to C++17.

[Lightsaver7]

Thank you for noticing this. I will update the docs and include this info in the next release.