rustdoc: support searching cfg predicates

[jyn514]

Currently, searching macros on https://docs.rs/tokio/1.33.0/tokio/ has 0 hits. It would be nice if it pulled up https://docs.rs/tokio/1.33.0/tokio/attr.main.html, which uses doc(cfg) to tell rustdoc that it's only available when the macros feature is enabled.

Note that this will make the search parser a little more complicated, since features don't have to be valid rust identifiers (in particular, it currently chokes on test-util):

@rustbot label F-doc_cfg

[Jules-Bertholet]

@rustbot label A-rustdoc-search C-feature-request T-rustdoc