Update Rust extension documentation to reference PR mongodb#2695

- Remove 'POC' and 'Copilot POC' references from comments
- Add comprehensive module-level documentation referencing PR mongodb#2695
- Document implementation history and performance characteristics
- Update function comments to be more descriptive and reference PR mongodb#2695

This reflects that the Rust BSON extension is a complete implementation
developed through PR mongodb#2695, not just a proof of concept.

Author: aclark4life

bson/_rbson/src/lib.rs

@@ -16,6 +16,22 @@
 //!
 //! This module provides the same interface as the C extension (bson._cbson)
 //! but implemented in Rust using PyO3 and the bson library.
+//!
+//! # Implementation History
+//!
+//! This implementation was developed as part of PYTHON-5683 to investigate
+//! using Rust as an alternative to C for Python extension modules.
+//!
+//! See PR #2695 for the complete implementation history, including:
+//! - Initial implementation with 100% test compatibility
+//! - Performance optimizations (type caching, fast paths, direct conversions)
+//! - Architectural analysis comparing Rust vs C extension approaches
+//!
+//! # Performance
+//!
+//! Current performance: ~0.21x (5x slower than C extension)
+//! Root cause: Architectural difference (Python ↔ Bson ↔ bytes vs Python ↔ bytes)
+//! See README.md for detailed performance analysis and optimization opportunities.
 
 #![allow(clippy::useless_conversion)]
 
@@ -398,7 +414,8 @@ fn str_flags_to_int(options: &str) -> i32 {
     flags
 }
 
-/// Test function for POC validation
+/// Test function to verify Rust extension is loaded
+/// Used by test suite to confirm PYMONGO_USE_RUST environment variable is working
 #[pyfunction]
 fn _test_rust_extension(py: Python) -> PyResult<PyObject> {
     let result = PyDict::new(py);
@@ -805,8 +822,9 @@ fn _dict_to_bson(
 ) -> PyResult<Py<PyBytes>> {
     let codec_options = Some(_codec_options);
 
-    // COPILOT POC APPROACH: Use python_mapping_to_bson_doc for better performance
+    // Use python_mapping_to_bson_doc for efficient encoding
     // This uses items() method and efficient tuple extraction
+    // See PR #2695 for implementation details and performance analysis
     let doc = python_mapping_to_bson_doc(obj, check_keys, codec_options, true)
         .map_err(|e| {
             // Match C extension behavior: TypeError for non-mapping types, InvalidDocument for encoding errors
@@ -831,7 +849,7 @@ fn _dict_to_bson(
             }
         })?;
 
-    // Use to_writer() to write directly to buffer (like Copilot POC)
+    // Use to_writer() to write directly to buffer
     // This is faster than bson::to_vec() which creates an intermediate Vec
     let mut buf = Vec::new();
     doc.to_writer(&mut buf)
@@ -1444,7 +1462,7 @@ fn _bson_to_dict(
 }
 
 /// Process a single item from a mapping's items() iterator
-/// COPILOT POC APPROACH: Efficient tuple extraction
+/// Uses efficient tuple extraction for performance
 fn process_mapping_item(
     item: &Bound<'_, PyAny>,
     doc: &mut Document,
@@ -1503,7 +1521,8 @@ fn process_mapping_item(
 }
 
 /// Convert a Python mapping (dict, SON, OrderedDict, etc.) to a BSON Document
-/// HYBRID APPROACH: Fast path for PyDict, Copilot POC approach for other mappings
+/// HYBRID APPROACH: Fast path for PyDict, items() method for other mappings
+/// See PR #2695 for implementation details and performance benchmarks
 fn python_mapping_to_bson_doc(
     obj: &Bound<'_, PyAny>,
     check_keys: bool,
@@ -1583,7 +1602,7 @@ fn python_mapping_to_bson_doc(
     }
 
     // SLOW PATH: Fall back to mapping protocol for SON, OrderedDict, etc.
-    // Use Copilot POC approach with items() method
+    // Use items() method for efficient iteration
     if let Ok(items_method) = obj.getattr("items") {
         if let Ok(items_result) = items_method.call0() {
             // Try to downcast to PyList or PyTuple first for efficient iteration