Clarify filesystem roots access scope

[tarunag10]

Summary

• clarify that omitting filesystem path args delegates allowed-directory scope to client-provided Roots
• note that the server waits for Roots during initialization rather than granting access on its own

Why

This addresses #4423 by making the argless quickstart/config behavior explicit for users configuring the filesystem server in MCP clients.

Tests

• Not run; documentation-only change
• git diff --check

Fixes #4423