doc: mention feature toggling in "Specifying deps"
doc: mention feature toggling in "Specifying deps"

tsurai/cargo,durka/cargo,achanda/cargo,eminence/cargo,AndiDog/cargo,sbeckeriv/cargo,achanda/cargo,sbeckeriv/cargo,rust-lang/cargo,durka/cargo,eminence/cargo,tsurai/cargo,tsurai/cargo,rustyhorde/cargo,tshepang/cargo,durka/cargo,AndiDog/cargo,vojtechkral/cargo,AndiDog/cargo,achanda/cargo,rust-lang/cargo,nipunn1313/cargo,tsurai/cargo,tshepang/cargo,vojtechkral/cargo,nipunn1313/cargo,nipunn1313/cargo,rustyhorde/cargo,vojtechkral/cargo,durka/cargo,tshepang/cargo,rust-lang/cargo,sbeckeriv/cargo,sbeckeriv/cargo,eminence/cargo,rustyhorde/cargo,eminence/cargo,rustyhorde/cargo,vojtechkral/cargo,achanda/cargo,tshepang/cargo,nipunn1313/cargo,AndiDog/cargo,rust-lang/cargo,tsurai/cargo,AndiDog/cargo