Some docs for std::num

This commit does two things:

* Re-works the module-level documentation.
* Cleaning up wording and adding links to where error types are used.

Part of #29364

src/libstd/num/mod.rs

@@ -8,10 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-//! Numeric traits and functions for generic mathematics
+//! Additional functionality for numerics.
 //!
-//! These are implemented for the primitive numeric types in `std::{u8, u16,
-//! u32, u64, usize, i8, i16, i32, i64, isize, f32, f64}`.
+//! This module provides some extra types that are useful when doing numerical
+//! work. See the individual documentation for each piece for more information.
 
 #![stable(feature = "rust1", since = "1.0.0")]
 #![allow(missing_docs)]