Document all feature, and show feature requirements

This is similar to what we do in geojson and elsewhere. Though I just
learned about doc_auto_cfg which makes this way easier by automatically
annotating any required items, rather than needing to try to sprinkle a
bunch of calls all over like:

    #[cfg_attr(docsrs, doc(cfg(feature = "geo-types")))]
    mod my_geo_types_module;

Cargo.toml

@@ -26,3 +26,7 @@ default = ["geo-types"]
 [[bench]]
 name = "parse"
 harness = false
+
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = ["--cfg", "docsrs"]

src/lib.rs

@@ -13,6 +13,11 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// The unstable `doc_auto_cfg` feature annotates documentation with any required cfg/features
+// needed for optional items. We set the `docsrs` config when building for docs.rs. To use it
+// in a local docs build, run: `cargo +nightly rustdoc --all-features -- --cfg docsrs`
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+
 //! The `wkt` crate provides conversions to and from the [WKT (Well Known Text)](https://en.wikipedia.org/wiki/Well-known_text_representation_of_geometry)
 //! geometry format.
 //!