rustdoc: add unions to whitelist of sidebar types
rustdoc: add unions to whitelist of sidebar types

This resolves #43405.

graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust