Document RFC 2539 (`#[cfg_attr(p, attr1, attr2)]`)

[Centril]

• Version: 1.33
• PR: stabilize cfg_attr_multi rust#57332
• Tracking issue: Tracking issue for RFC 2539, "#[cfg_attr] expanding to multiple attributes" rust#54881
• RFC: #[cfg_attr] expanding to multiple attributes rfcs#2539