[Bug] failed command expression is not sending output to variable

The docs for [command-expressions](https://docs.amber-lang.com/basic_syntax/commands) say:
"Command expression result is sent to the variable instead of standard output."

This is not true, by running the following code
```ab
let result = $cat not_existent.txt$ failed {
echo "Failed to read the file"
}
echo "---"
echo result
```
You can see that the message is actually sent to standard output and is not saved in the result variable.

Is this an error in the amber language or an error in the docs?