Fix and improve MCP server security warnings in docs:

• First, fix the repeated mention to MCPClient in the same sentence

  The same security warnings mentioned for MCPClient apply when using MCPClient

• Restructure warnings using the inverted pyramid approach (most critical information first: call to action)
• Make all warnings self-contained rather than referencing previous warning
  • Ensure readers who skip to this section get complete information
  • Remove the need to search for previous context

These changes follow documentation best practices by ensuring critical security information is immediately visible and complete regardless of how users navigate the documentation.