add documentation, remove Map.as_target

src/latlng.mli

@@ -2,14 +2,22 @@
 
 type t
 
+(** [create lat lng] is an object representing a geographical point with the
+    given latitude and longitude *)
 val create : float -> float -> t
 
+(** [lat o] is the latitude of [o] *)
 val lat : t -> float
 
+(** [lng o] is the longitude of [o] *)
 val lng : t -> float
 
+(** [equals a b] is true iff [a] and [b] is at the same position (within a small
+    margin of error) *)
 val equals : t -> t -> bool
 
+(** [of_jv jv] is [jv] as a [Latlng.t] *)
 val of_jv : Jv.t -> t
 
+(** [to_jv o] is [o] as a {!Jv.t} *)
 val to_jv : t -> Jv.t