Use rust-bitcoin module doc style

Recently we introduced uniform styling for module docs over in
`rust-bitcoin` repo. We can do the same here but its a bit controversial
because it removes the heading from module docs and every single public
module in rust-secp256k1 uses a heading. Instead we use a full
sentences. Also makes uniform the trailing `//!`.

src/constants.rs

@@ -13,8 +13,8 @@
 // If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
 //
 
-//! # Constants
 //! Constants related to the API and the underlying curve.
+//!
 
 /// The size (in bytes) of a message.
 pub const MESSAGE_SIZE: usize = 32;

src/ecdh.rs

@@ -12,7 +12,6 @@
 // If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
 //
 
-//! # ECDH
 //! Support for shared secret computations.
 //!
 

src/ecdsa/recovery.rs

@@ -13,9 +13,9 @@
 // If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
 //
 
-//! # Recovery module
 //! Provides a signing function that allows recovering the public key from the
 //! signature.
+//!
 
 use core::ptr;
 use key;

src/lib.rs

@@ -13,7 +13,6 @@
 // If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
 //
 
-//! # Secp256k1
 //! Rust bindings for Pieter Wuille's secp256k1 library, which is used for
 //! fast and accurate manipulation of ECDSA signatures on the secp256k1
 //! curve. Such signatures are used extensively by the Bitcoin network