Build docs without optional impls
Build docs without optional impls

There's no way to identify them as opt-in in the docs which is a
constant source of confusion so it's safer to just not mention them
there.

achanda/rust-postgres,tempbottle/rust-postgres,achanda/rust-postgres,ddrmanxbxfr/rust-postgres,sfackler/rust-postgres,sfackler/rust-postgres,tempbottle/rust-postgres,ddrmanxbxfr/rust-postgres