units: Improve re-exports

Make an attempt to improve the ergonomics and docs clarity of the
`units` crate.

- Don't inline error type re-exports, this keeps them up in the
"Re-exports" section and saves cluttering the other inlined docs.

- Re-export and inline the docs for `FeeRate` and `Weight` same as we do
for `Amount`. This makes the "Structs" section of the docs nice except
for the exclusion of the locktime types (which cannot be helped).

units/src/lib.rs

@@ -46,9 +46,16 @@ pub mod parse;
 pub mod weight;
 
 #[doc(inline)]
-pub use self::amount::{Amount, ParseAmountError, SignedAmount};
+pub use self::amount::{Amount, SignedAmount};
+pub use self::amount::ParseAmountError;
 #[cfg(feature = "alloc")]
 pub use self::parse::ParseIntError;
+#[cfg(feature = "alloc")]
+#[doc(inline)]
+pub use self::{
+    fee_rate::FeeRate,
+    weight::Weight,
+};
 
 #[rustfmt::skip]
 #[allow(unused_imports)]