Improve CLI by documenting the expected format of SYMBOL

Some CLI commands of hiedb expect SYMBOL as an input.
For example
```
Usage: hiedb unreachable SYMBOL
Find all symbols unreachable from the given symbols
```
But when I try to use it I get
```
$ hiedb unreachable Main.main
cannot parse value `Main.main'

Usage: hiedb unreachable SYMBOL
Find all symbols unreachable from the given symbols
```

Looking at source code it's clear to me that the parsing is based on [Read instance of Symbol](https://github.com/wz1000/HieDb/blob/de8aec9984266f004a13b3c6b4d02d500ef99cd9/src/HieDb/Run.hs#L183). But it's totally non-obvious how to format the SYMBOL when querrying from CLI. Could you add an example of the expected format to cli help message that gets printed?