chore: Add docstring to clarify the behavior of preload memory tool

PiperOrigin-RevId: 793840979

Author: DeanChensj

src/google/adk/tools/preload_memory_tool.py

@@ -29,6 +29,9 @@
 class PreloadMemoryTool(BaseTool):
   """A tool that preloads the memory for the current user.
 
+  This tool will be automatically executed for each llm_request, and it won't be
+  called by the model.
+
   NOTE: Currently this tool only uses text part from the memory.
   """