Outdated/incomplete Python docs

[alecandido]

Since @RoyStegeman started using it, we finally got another user that is not a developer (first one being @scarlehoff). Nice chance to improve docs.

• [ ] convolute does not exist any longer (it is called convolute_with_one) https://github.com/N3PDF/pineappl/blob/552dff4780ce5e4c0e037474391e662bb92ef54c/pineappl_py/docs/source/recipes.rst#L16
• [ ] the call is also outdated: now the first argument now should be pdg_id (same call)