Small project updates

- bumped dune and opam versions
- Added synopsis in opam file
- small doc adjustments

lib/libusb.mli

@@ -6,12 +6,11 @@
 
     I wrote this alternate binding:
     - in order to experiment more in depth with {{:https://github.com/ocamllabs/ocaml-ctypes} Ctypes};
-    - and because I needed something more portable than {{:https://github.com/letoh/ocaml-usb} ocaml-usb}: this one is working on Windows. *)
+    - and because I needed something more portable than {{:https://github.com/letoh/ocaml-usb} ocaml-usb}: this one is working on Windows, which was not the case of ocaml-usb when I started this project. *)
 
 include module type of T
 
-
-(** {1 Getting more of {! error}s} *)
+(** {1 Getting more of errors} *)
 
 val string_of_error: error -> string
 
@@ -44,7 +43,7 @@ val init_libusb: unit -> (unit, error) result
 (** This function must be called before calling any other libusb function. *)
 
 val exit_libusb: unit -> unit
-(** Should be called after closing all open devices and before your application
+(** Should be called after closing all opened devices and before your application
    terminates. *)
 
 val get_version: unit -> version
@@ -65,7 +64,7 @@ val unref_device: device -> unit
 (** [unref_device d] decrements the reference count of [d].
 
     If the decrement operation causes the reference count to reach zero, the
-   device shall be destroyed by libusb C library. *)
+    device shall be destroyed by libusb C library. *)
 
 val unref_devices: device list -> unit
 (** [unref_devices l == List.iter unref_device l]  *)