Making docs accessible to Python only and / or Rust only.

At the moment if we have a function like the following:

/// A function to add two numbers together.
///
/// # Examples
///
/// ```rust
/// let x = sum(10, 12);
/// assert_eq!(x, 22);
/// ```
/// 
/// ``` python
/// >>> assert sum(10, 12) == 22
/// ```
///
/// # Errors
/// 
/// This function panics on overflow of `i32`.
#[pyfunction]
fn sum(x: i32, y: i32) -> i32 { ... }

Then we currently copy the whole docstring from Rust to Python. This is not ideal; it would be nice to be able to have a Rust-specific example which is only visible from rustdoc and a Python-specific example which is only visible from Python.

I think we might be able to solve this by having a #[pyo3(doc)] attribute which we can use to switch mode for consuming docs into the final form for each language.

I think it might be something like:

• #[pyo3(doc = "rust")] - docs from this point onward are left in-place for Rustdoc and stripped from Python docstring
• #[pyo3(doc = "py")] - docs from this point onward are stripped from Rustdoc and included in the Python docstring. I think we can do this by manipulating the expanded #[doc] attributes, but needs confirmation.
• #[pyo3(doc = "both")] - docs from this point onwards are visible in both languages. This is the default mode all docs start in.

So the above might be written something like:

/// A function to add two numbers together.
///
/// # Examples
///
#[pyo3(doc = "rust")] // rust-only example
/// ```rust
/// let x = sum(10, 12);
/// assert_eq!(x, 22);
/// ```
///
#[pyo3(doc = "py")]  // python-only example
/// ``` python
/// >>> assert sum(10, 12) == 22
/// ```
///
#[pyo3(doc = "both")]  // show the errors section in both languages
/// # Errors
/// 
/// This function panics on overflow of `i32`.
#[pyfunction]
fn sum(x: i32, y: i32) -> i32 { ... }

What do folks think?

There might be better forms like #[pyo3(doc(rust))] / #[pyo3(doc(py))] etc. If the concept is proven to work we can bikeshed the syntax.

Start by tracing how PyO3 copies Rust doc comments into Python docstrings and how expanded #[doc] attributes are handled. Confirm whether language-specific sections can be removed from one generated form while remaining in the other. Done means a documented syntax supports rust-only, Python-only, and shared sections, with tests covering both generated documentation outputs.