docs: clarify that MCP tools require glob patterns to disable

[3mdistal]

Summary

• Adds a note explaining that MCP tools are registered with names like server-name_tool-name
• Clarifies why glob patterns (e.g., "my-mcp*") are needed instead of exact server names

Related issues: #5373, #2953

Users setting "my-mcp": false in their tools config expect it to disable all tools from that MCP server, but it doesn't work because the actual tool names are prefixed (e.g., my-mcp_search). This note makes the naming convention explicit.