Provide a practical map_coords example up-front

These traits can seem a little abstract without examples, and though
we provide at least one for each, providing one at module level might
make it easier to grasp what you can do with it.

geo/src/algorithm/map_coords.rs

@@ -1,3 +1,39 @@
+//! # Advanced Example: Fallible Geometry coordinate conversion using `PROJ`
+//!
+//! ```
+//! // activate the [use-proj] feature in cargo.toml in order to access proj functions
+//! # #[cfg(feature = "use-proj")]
+//! use geo::{Coordinate, Point};
+//! # #[cfg(feature = "use-proj")]
+//! use geo::algorithm::map_coords::TryMapCoords;
+//! # #[cfg(feature = "use-proj")]
+//! use proj::Proj;
+//! // GeoJSON uses the WGS 84 coordinate system
+//! # #[cfg(feature = "use-proj")]
+//! let from = "EPSG:4326";
+//! // The NAD83 / California zone 6 (ftUS) coordinate system
+//! # #[cfg(feature = "use-proj")]
+//! let to = "EPSG:2230";
+//! # #[cfg(feature = "use-proj")]
+//! let to_feet = Proj::new_known_crs(&from, &to, None).unwrap();
+//! # #[cfg(feature = "use-proj")]
+//! let f = |x: f64, y: f64| {
+//!     // proj can accept Point, Coordinate, Tuple, and array values, returning a Result
+//!     let shifted = to_feet.convert((x, y))?;
+//!     Ok((shifted.x(), shifted.y()))
+//! };
+//! # #[cfg(feature = "use-proj")]
+//! // 👽
+//! # #[cfg(feature = "use-proj")]
+//! let usa_m = Point::new(-115.797615, 37.2647978);
+//! # #[cfg(feature = "use-proj")]
+//! let usa_ft = usa_m.try_map_coords(|&(x, y)| f(x, y)).unwrap();
+//! # #[cfg(feature = "use-proj")]
+//! assert_eq!(6693625.67217475, usa_ft.x());
+//! # #[cfg(feature = "proj")]
+//! assert_eq!(3497301.5918027186, usa_ft.y());
+//! ```
+
 use crate::{
     Coordinate, CoordinateType, Geometry, GeometryCollection, Line, LineString, MultiLineString,
     MultiPoint, MultiPolygon, Point, Polygon, Rect,
@@ -103,7 +139,7 @@ pub trait TryMapCoords<T, NT> {
         NT: CoordinateType;
 }
 
-/// Map all the coordinates in an object in place
+/// Map a function over all the coordinates in an object in place
 pub trait MapCoordsInplace<T> {
     /// Apply a function to all the coordinates in a geometric object, in place
     ///