units: Fix error re-exports

Currently we re-export two error types at the crate root, this is
surprising because:

- Why not none or all the rest?
- Why these two?

Observe that the `ParseIntError` is special in that it is used by
other modules so its good to have at the crate root (other errors are
expected to be used with a module prefix eg, `amount::ParseError`).

There is no obvious reason why `ParseAmountError` is re-exported.

Comment and doc inline the `ParesIntError`, remove the re-export of
the `ParseAmountError`.

units/src/lib.rs

@@ -45,10 +45,11 @@ pub mod parse;
 #[cfg(feature = "alloc")]
 pub mod weight;
 
-pub use self::amount::ParseAmountError;
 #[doc(inline)]
 pub use self::amount::{Amount, SignedAmount};
+// ParseIntError is used by other modules, so we re-export it.
 #[cfg(feature = "alloc")]
+#[doc(inline)]
 pub use self::parse::ParseIntError;
 #[cfg(feature = "alloc")]
 #[doc(inline)]