docs: Add comprehensive docstrings and fix default top_k consistency

Address PR feedback from CodeRabbit AI:
- Add detailed docstring for search_task internal function
- Document top_k parameter in main search function docstring
- Fix default top_k inconsistency (was 10 in client, now 5 everywhere)
- Clarify performance implications of different top_k values

Changes:
- server.py: Add top_k parameter documentation and search_task docstring
- cognee_client.py: Change default top_k from 10 to 5 for consistency

This ensures consistent behavior across the MCP call chain and
provides clear guidance for users on choosing appropriate top_k values.

cognee-mcp/src/cognee_client.py

@@ -151,7 +151,7 @@ class CogneeClient:
         query_type: str,
         datasets: Optional[List[str]] = None,
         system_prompt: Optional[str] = None,
-        top_k: int = 10,
+        top_k: int = 5,
     ) -> Any:
         """
         Search the knowledge graph.

cognee-mcp/src/server.py

@@ -389,6 +389,13 @@ async def search(search_query: str, search_type: str, top_k: int = 5) -> list:
 
         The search_type is case-insensitive and will be converted to uppercase.
 
+    top_k : int, optional
+        Maximum number of results to return (default: 5).
+        Controls the amount of context retrieved from the knowledge graph.
+        - Lower values (3-5): Faster, more focused results
+        - Higher values (10-20): More comprehensive, but slower and more context-heavy
+        Helps manage response size and context window usage in MCP clients.
+
     Returns
     -------
     list
@@ -426,7 +433,26 @@ async def search(search_query: str, search_type: str, top_k: int = 5) -> list:
     """
 
     async def search_task(search_query: str, search_type: str, top_k: int) -> str:
-        """Search the knowledge graph"""
+        """
+        Internal task to execute knowledge graph search with result formatting.
+
+        Handles the actual search execution and formats results appropriately
+        for MCP clients based on the search type and execution mode (API vs direct).
+
+        Parameters
+        ----------
+        search_query : str
+            The search query in natural language
+        search_type : str
+            Type of search to perform (GRAPH_COMPLETION, CHUNKS, etc.)
+        top_k : int
+            Maximum number of results to return
+
+        Returns
+        -------
+        str
+            Formatted search results as a string, with format depending on search_type
+        """
         # NOTE: MCP uses stdout to communicate, we must redirect all output
         #       going to stdout ( like the print function ) to stderr.
         with redirect_stdout(sys.stderr):