Properly handle cfgs in rustdoc

Rustdoc would previously improperly handle key="value" style cfgs, which
are notably used for Cargo features.
auto merge of #17834 : sfackler/rust/rustdoc-cfgs, r=alexcrichton,alexcrichton

Rustdoc would previously improperly handle key="value" style cfgs, which
are notably used for Cargo features.