Improve rustdocs on error type

Improve rustdocs on `Error` by doing:

- Document the reason we use unconventional enum variant identifiers.
- Use a live link to `shared-libraries.md` in the Bitcoin Core repo
- Use angle brackets on link

Author: tcharding

src/lib.rs

@@ -11,7 +11,11 @@ use core::fmt;
 
 use crate::types::*;
 
-/// Errors returned by `libbitcoinconsensus` (see github.com/bitcoin/bitcoin/doc/shared-libraries.md).
+/// Errors returned by [`libbitcoinconsensus`].
+///
+/// The error variant identifiers mimic those from `libbitcoinconsensus`.
+///
+/// [`libbitcoinconsensus`]: <https://github.com/bitcoin/bitcoin/blob/master/doc/shared-libraries.md#errors>
 #[allow(non_camel_case_types)]
 #[derive(Debug)]
 #[repr(C)]