Add crate comment for py_ext.

crates/accelerate/src/quantum_circuit/py_ext.rs

@@ -9,6 +9,17 @@
 // Any modifications or derivative works of this code must retain this
 // copyright notice, and modified files need to carry a notice indicating
 // that they have been altered from the originals.
+//! Contains helper functions for creating [Py<T>] (GIL-independent)
+//! objects without creating an intermediate owned reference. These functions
+//! are faster than PyO3's list and tuple factory methods when the caller
+//! doesn't need to dereference the newly constructed object (i.e. if the
+//! resulting [Py<T>] will simply be stored in a Rust struct).
+//!
+//! The reason this is faster is because PyO3 tracks owned references and
+//! will perform deallocation when the active [GILPool] goes out of scope.
+//! If we don't need to dereference the [Py<T>], then we can skip the
+//! tracking and deallocation.
+
 use pyo3::ffi::Py_ssize_t;
 use pyo3::prelude::*;
 use pyo3::types::{PyList, PyTuple};