Use doc_cfg feature instead of doc_auto_cfg on docsrs

[ids1024]

Fixes build with recent nightly, as now used by docs.rs.

Can be tested with RUSTDOCFLAGS=--cfg=docsrs cargo +nightly doc --all-features, with these patches:

[patch.crates-io]
cursor-icon = { git = "https://github.com/rust-windowing/cursor-icon" }
wayland-client = { git = "https://github.com/smithay/wayland-rs" }
wayland-protocols = { git = "https://github.com/smithay/wayland-rs" }
wayland-protocols-wlr = { git = "https://github.com/smithay/wayland-rs" }
wayland-protocols-misc = { git = "https://github.com/smithay/wayland-rs" }
wayland-cursor = { git = "https://github.com/smithay/wayland-rs" }
wayland-backend = { git = "https://github.com/smithay/wayland-rs" }
wayland-csd-frame = { git = "https://github.com/ids1024/wayland-csd-frame", branch = "docsrs" }

As well as updating the wayland-protocols-experimental dependency to git = "https://github.com/smithay/wayland-rs". (Though presumably other changes may be needed to correctly update experimental protocols.)