docs: clarify append_context_keys behavior with overlapping keys

Fixes #7690

Clarifies that append_context_keys() removes all added keys on exit,
including keys that already existed with the same name. This addresses
user confusion about the 'temporarily' behavior when keys overlap.

Changes:
- Updated docstrings in formatter.py and logger.py with clear warning
- Added detailed documentation in logger.md with collision example
- Explains that original values are lost when keys overlap
- Recommends using append_keys() for persistent keys to avoid collisions

This is a documentation-only fix that clarifies current behavior without
changing functionality.

Author: oyiz-michael

aws_lambda_powertools/logging/formatter.py

@@ -281,6 +281,12 @@ def append_context_keys(self, **additional_keys: Any) -> Generator[None, None, N
         **additional_keys: Any
             Key-value pairs to include in the log context during the lifespan of the context manager.
 
+        Warning
+        -------
+        All keys added within this context are removed when exiting, even if they existed before.
+        If a key with the same name already exists, the original value will be lost after the context exits.
+        To persist keys across multiple log messages, use `append_keys()` instead.
+
         Example
         --------
             logger = Logger(service="example_service")

aws_lambda_powertools/logging/logger.py

@@ -820,6 +820,12 @@ def append_context_keys(self, **additional_keys: Any) -> Generator[None, None, N
         **additional_keys: Any
             Key-value pairs to include in the log context during the lifespan of the context manager.
 
+        Warning
+        -------
+        All keys added within this context are removed when exiting, even if they existed before.
+        If a key with the same name already exists, the original value will be lost after the context exits.
+        To persist keys across multiple log messages, use `append_keys()` instead.
+
         Example
         --------
         **Logging with contextual keys**

docs/core/logger.md

@@ -195,6 +195,22 @@ You can append your own keys to your existing Logger via `append_keys(**addition
 
 The append_context_keys method allows temporary modification of a Logger instance's context without creating a new logger. It's useful for adding context keys to specific workflows while maintaining the logger's overall state and simplicity.
 
+???+ danger "Important: Keys are removed on context exit, even if they existed before"
+	All keys added within the context are removed when exiting, **including keys that already existed with the same name**. 
+	
+	If you need to temporarily override a key's value while preserving the original, use `append_keys()` for persistent keys and avoid key name collisions with `append_context_keys()`.
+	
+	**Example of key collision:**
+	```python
+	logger.append_keys(order_id="ORD-123")  # Persistent key
+	logger.info("Order received")  # Has order_id="ORD-123"
+	
+	with logger.append_context_keys(order_id="ORD-CHILD"):  # Overwrites
+	    logger.info("Processing")  # Has order_id="ORD-CHILD"
+	
+	logger.info("Order completed")  # order_id key is now MISSING!
+	```
+
 === "append_context_keys.py"
 
     ```python hl_lines="7 8"