Misleading documentation

### Expected Behavior
The [documentation for within()](https://google.github.io/zx/api#within) does not use `cd()` due to it touching `process.cwd` and not reverting in some way, but having no mention of this side-effect as part of `within()`

### Actual Behavior
The [documentation for within()](https://google.github.io/zx/api#within) uses `cd()` which changes the `process.cwd` of the entire script and introduces parity between `process.cwd` and `` $`cd` ``

### Steps to Reproduce

1. Run this code for breaking example:
```js
echo("Script start");
echo(`- ${await $`pwd`}`); // => cwd

await within(async () => {
echo("First within");
cd("/");
echo(`- ${await $`pwd`}`); // => /
});

echo("Between within");
echo(`- ${await $`pwd`}`); // => cwd

await within(async () => {
echo("Second within");
echo(`- process CWD: ${process.cwd()}`); // => /
echo(`- ${await $`pwd`}`); // => cwd
});

echo("Script end");
echo(`- ${await $`pwd`}`); // => cwd
```

### Specifications

- zx version: 8.1.4
- Platform: macos
- Runtime: node