Fix recipe docs for probes

[halfflat]

As per the title, the C++ API documentation for recipe is out of date. So we should update it.

[halfflat]

Documentation structure is currently in flux; we will return to this after reorg.

[thorstenhater]

Docs have somewhat stabilised. @brenthuisman want to this up?

[brenthuisman]

doc/cpp/probe_sample.rst exists and seems quite complete. I even see some definitions that didn't get a glossary directive, so were not seen in the recent update.

Let's close.