docs: When building for docs.rs, add feature flag info

When docs.rs builds docs, it builds with nightly Rust and the
cfg `docsrs` set. We can trigger on `cfg(docsrs)` to enable
an unstable nightly feature to show which features are
required to enable something in the generated docs.

fidget/src/lib.rs

@@ -262,6 +262,7 @@
 //!
 //! # Feature flags
 #![doc = document_features::document_features!()]
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
 #![warn(missing_docs)]
 
 // Re-export everything from fidget::core into the top-level namespace