Update library inline docs descriptions (#5984)

## Description

While reviewing the [std-lib
docs](https://fuellabs.github.io/sway/master/std/), it was noted that
some of the libraries and modules do not have docs or are outdated.
These have been updated.

## Checklist

- [ ] I have linked to any relevant issues.
- [ ] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [ ] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [ ] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

---------

Co-authored-by: K1-R1 <77465250+K1-R1@users.noreply.github.com>

sway-lib-std/src/address.sw

@@ -1,4 +1,4 @@
-//! A wrapper around the `b256` type to help enhance type-safety.
+//! The `Address` type used for interacting with addresses on the fuel network.
 library;
 
 use ::convert::From;

sway-lib-std/src/alias.sw

@@ -1,3 +1,4 @@
+//! Aliases for various types in Sway.
 library;
 
 /// The `SubId` type is simply an alias for `b256` that represents the sub identifier of a native asset.

sway-lib-std/src/asset_id.sw

@@ -1,3 +1,4 @@
+//! The `AssetId` type used for interacting with an asset on the fuel network.
 library;
 
 use ::alias::SubId;

sway-lib-std/src/b512.sw

@@ -1,5 +1,4 @@
-//! A wrapper around two `b256` types to support the usage of 64-byte values in Sway,
-//! which are needed when working with public keys and signatures.
+//! The `B512` type supports the usage of 64-byte values in Sway which are needed when working with public keys and signatures.
 library;
 
 use ::constants::ZERO_B256;

sway-lib-std/src/constants.sw

@@ -1,4 +1,4 @@
-//! Base asset and zero address constants.
+//! Definitions for constant values in Sway.
 library;
 
 /// A b256 of zero value.

sway-lib-std/src/contract_id.sw

@@ -1,4 +1,4 @@
-//! A wrapper around the `b256` type to help enhance type-safety.
+//! The `ContractId` type used for interacting with contracts on the fuel network.
 library;
 
 use ::convert::From;

sway-lib-std/src/execution.sw

@@ -1,3 +1,4 @@
+//! Helper functions to load and run external contract code.
 library;
 
 use ::contract_id::ContractId;

sway-lib-std/src/iterator.sw

@@ -1,3 +1,4 @@
+//! The iterator trait to iterate over elements.
 library;
 
 use ::option::Option::{self, *};

sway-lib-std/src/string.sw

@@ -1,3 +1,4 @@
+//! A UTF-8 encoded growable string.
 library;
 
 use ::assert::assert;