[mlir:python] Improve `mlir_(attribute|type|value)_subclass` for `nanobind`s `stubgen` (#127584)

This PR makes several improvements to the stubs that are created by
`mlir_(attribute|type|value)_subclass`.

First, the PR sets the `__module__` attribute of the classes generated
by the nanobind adaptors for attributes, types, and values (via
`mlir_(attribute|type|value)_subclass`). By default, the `__module__`
property is set to `importlib._bootstrap`, which isn't where we want the
new class to live. The new logic sets the property to the name of the
module provided as `scope` instead. This also makes nanobind's `stubgen`
generate stubs for those classes properly, which ignores classes whose
`__module__` does not correspond to the module it is generating stubs
for. This resolves #127518.

Second, the PR overwrites the function signatures generated by `stubgen`
to a format that uses the desired type names (e.g., `mlir.ir.Attribute`
instead of `MlirAttribute`).

Finally, the PR piggy-backs some minor doc and style improvements to
`PythonAdaptors.h`.

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Signed-off-by: Ingo Müller <ingomueller@google.com>