Update the documentation of the option `ignore-expiry` to make clear what it actually does

We need to revise the documentation of the cabal.project option `ignore-expiry` to prevent confusion. Setting this option will disable the verification of remote repositories' signature expiration on the command execution of `cabal update`, and the flag has nothing to do with the following familiar warning:

```
Warning: The package list for 'hackage.haskell.org' is *** days old.
Run 'cabal update' to get the latest list of available packages.
```

In fact, I, myself, misunderstood what the option does. This issue, as it was first written, complained that the option didn't do what I expected it to do! This issue has been updated through conversation to reflect reality. I attach below my embarrassingly mistaken original issue.

---
### The initial issue-opening comment. This is based on my misunderstanding.

**Describe the bug**
The `ignore-expiry` option doesn't have implementation in this codebase.
Therefore, our familiar warning
```
Warning: The package list for 'hackage.haskell.org' is *** days old.
Run 'cabal update' to get the latest list of available packages.
```
will not go away with `cabal --ignore-expiry`.
However this lack of implementation does not seem to be documented.

The single function `readRepoIndex`, implemented in [lines 363-415 of `Distribution.Client.IndexUtils` in `cabal-install`, `master` as of 2023-04-25T01:37:14Z (`5f77754`)](https://github.com/haskell/cabal/blob/5f77754c5ed6037e04022b03b1bb0a5e46acecd6/cabal-install/src/Distribution/Client/IndexUtils.hs#L363-L415), have the total control of the warning. This function does not have access to the status of `ignore-expiry` option! Furthermore, [the search against this repository](https://github.com/search?q=repo%3Ahaskell%2Fcabal%20globalIgnoreExpiry&type=code) does not reveal any code that inspects the field `globalIgnoreExpiry` of the data type `GlobalFlags` (defined in [lines 58-73 of `Distribution.Client.GlobalFlags`](https://github.com/haskell/cabal/blob/5f77754c5ed6037e04022b03b1bb0a5e46acecd6/cabal-install/src/Distribution/Client/GlobalFlags.hs#L58-L73)); this strongly suggests that `ignore-expiry` option has completely no influence on the behavior of `cabal-install`.

**To Reproduce**
Steps to reproduce the behavior: for example,

```console
$ REPOCACHE=[your remote-repo-cache value, e.g. $HOME/.cabal/packages]/hackage.haskell.org/01-index.tar
$ touch -d 2000-01-01T00:00:00Z $REPOCACHE
$ cabal --ignore-expiry info hello
Warning: The package list for 'hackage.haskell.org' is 8515 days old.
Run 'cabal update' to get the latest list of available packages.
* hello (program)
Synopsis: Hello World, an example package
Versions available: 1.0, 1.0.0.1, 1.0.0.2
Versions installed: [ Unknown ]
Homepage: http://www.haskell.org/hello/
Bug reports: mailto:marlowsd@gmail.com
Description: This is an implementation of the classic "Hello World"
program in Haskell, as an example of how to create a minimal
Haskell application using Cabal and Hackage. Please submit
any suggestions and improvements.
Category: Console, Text
License: BSD3
Author: Simon Marlow
Maintainer: Simon Marlow
Source repo: http://darcs.haskell.org/hello/
Executables: hello
Flags: threaded
Dependencies: base >=4.2 && <5
Cached: No
```

**Expected behavior**

For example, the shell session above should be changed to:

```console
$ REPOCACHE=[your remote-repo-cache value, e.g. $HOME/.cabal/packages]/hackage.haskell.org/01-index.tar
$ touch -d 2000-01-01T00:00:00Z $REPOCACHE
$ cabal --ignore-expiry info hello
* hello (program)
Synopsis: Hello World, an example package
Versions available: 1.0, 1.0.0.1, 1.0.0.2
Versions installed: [ Unknown ]
Homepage: http://www.haskell.org/hello/
Bug reports: mailto:marlowsd@gmail.com
Description: This is an implementation of the classic "Hello World"
program in Haskell, as an example of how to create a minimal
Haskell application using Cabal and Hackage. Please submit
any suggestions and improvements.
Category: Console, Text
License: BSD3
Author: Simon Marlow
Maintainer: Simon Marlow
Source repo: http://darcs.haskell.org/hello/
Executables: hello
Flags: threaded
Dependencies: base >=4.2 && <5
Cached: No
```

**System information**
- Operating system: MacOS 13.3.1（22E261）
- `cabal`: 3.8.1.0 OR 3.10.1.0. Absence of implementation confirmed on [`master` as of 2023-04-25T01:37:14Z (`5f77754`)](https://github.com/haskell/cabal/commit/5f77754c5ed6037e04022b03b1bb0a5e46acecd6).
- `ghc`: 9.4.4

**Additional context**
I'm writing a Haskell build environment for the judge server of a competitive programming website [AtCoder](https://atcoder.jp/home).
The server forces an offline build for each submission; we need to download and set up all dependencies on the installation time, so this feature is absolutely desirable!