docs: Add docstrings for CassandraChatMessageHistory class and package namespace function. (#24222)

- Modified docstring for CassandraChatMessageHistory in
libs/community/langchain_community/chat_message_history/cassandra.py.

- Added docstring for _package_namespace function in
docs/api_reference/create_api_rst.py

---------

Co-authored-by: ashvin <ashvin.anilkumar@qburst.com>
Co-authored-by: Erick Friis <erick@langchain.dev>
Co-authored-by: ccurme <chester.curme@gmail.com>

docs/api_reference/create_api_rst.py

@@ -471,6 +471,14 @@ def _build_rst_file(package_name: str = "langchain") -> None:
 
 
 def _package_namespace(package_name: str) -> str:
+    """Returns the package name used.
+
+    Args:
+        package_name: Can be either "langchain" or "core" or "experimental".
+
+    Returns:
+        modified package_name: Can be either "langchain" or "langchain_{package_name}"
+    """
     return (
         package_name
         if package_name == "langchain"

libs/community/langchain_community/chat_message_histories/cassandra.py

@@ -31,6 +31,8 @@ def _rows_to_messages(rows: Iterable[RowType]) -> List[BaseMessage]:
 
 
 class CassandraChatMessageHistory(BaseChatMessageHistory):
+    """Chat message history that is backed by Cassandra."""
+
     def __init__(
         self,
         session_id: str,
@@ -41,7 +43,8 @@ class CassandraChatMessageHistory(BaseChatMessageHistory):
         *,
         setup_mode: SetupMode = SetupMode.SYNC,
     ) -> None:
-        """Chat message history that stores history in Cassandra.
+        """
+        Initialize a new instance of CassandraChatMessageHistory.
 
         Args:
             session_id: arbitrary key that is used to store the messages