Review conversions and ensure consistency

Continued from https://github.com/PyO3/pyo3/pull/3197#discussion_r1225910628

We would like to ensure that we're consistent in how conversions from Python objects to Rust objects select for what's allowed.

I think the right approach would be to do something like https://github.com/PyO3/pyo3/blob/main/guide/src/conversions/tables.md where for each Rust type we state what Python types will convert to it (i.e. FromPyObject), and what Python type(s) it will convert back into (i.e. IntoPy<PyObject>).

That information does live on that page, but perhaps it can benefit from being rearranged.

In general we seem to be preferring conversions which are allow the user to be a bit loose with typing (e.g. string for IP address). So I think we should ensure we stick to this philosophy everywhere (e.g. maybe we should accept int for bool, or even all truthy types)?

We may also wish to consider a "strict" mode where only the exact Python type can be input (e.g. Python bool <-> Rust bool).

Start with the discussion in pull request 3197 and the conversion table at guide/src/conversions/tables.md. Review the documented FromPyObject and IntoPy behavior for each Rust type, then identify inconsistencies in accepted Python types and whether a strict mode is within scope. Done means the conversion policy is agreed and consistently documented or implemented.