Document RFC 2103 `tool_attributes`

[ehuss]

• Version: 1.30
• Stabilization: Stabilize a few secondary macro features rust#53459
• Tracking: Tracking issue: RFC 2103 - attributes for tools rust#44690
• RFC: https://github.com/rust-lang/rfcs/blob/master/text/2103-tool-attributes.md

Tool lint attributes are documented, but general tool attributes are not.