Update ADK doc according to issue #1521 - 28

docs/tools/retrieval/files-retrieval.md

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+# FilesRetrieval
+
+The `FilesRetrieval` tool allows an agent to perform retrieval over a local directory of files.
+
+## Initialization
+
+To use the `FilesRetrieval` tool, you need to initialize it by providing the path to the directory containing the files you want to use for retrieval. You specify this directory using the `input_dir` parameter.
+
+The default embedding model used by the `FilesRetrieval` tool is `gemini-embedding-2-preview`.
+
+## Example
+
+The following example shows how to initialize and use the `FilesRetrieval` tool:
+
+```python
+from google.adk.tools.retrieval import FilesRetrieval
+
+# Create an instance of the FilesRetrieval tool
+files_retrieval_tool = FilesRetrieval(
+    name="files_retrieval",
+    description="Performs retrieval over a local directory of files.",
+    input_dir="/path/to/your/files"
+)
+
+# Now you can use the tool to retrieve information from the files
+# in the specified directory.
+```