Make `rails runner` command options more obvious

We're surrounding the options in angle brackets `<>` as is convention in `curl`:

```
$ curl --help
Usage: curl [options...] <url>
```

And then in square brackets `[]` with bars `|` as in `tar`:

```
$ tar --help
...
Create: tar -c [options] [<file> | <dir> | @<archive> | -C <dir> ]
```

To further clarify that the command can be used with both, we now show examples:

```
Examples:
    rails runner 'puts Rails.env'
        This runs the code `puts Rails.env` after loading the app

    rails runner path/to/filename.rb
        This runs the Ruby file located at `path/to/filename.rb` after loading the app
```

This format was taken from the `find` man pages:

```
EXAMPLES
     The following examples are shown as given to the shell:

     find / \! -name "*.c" -print
             Print out a list of all the files whose names do not end in .c.

     find / -newer ttt -user wnj -print
             Print out a list of all the files owned by user ``wnj'' that are newer than the file ttt.
```

The the text at the bottom is also shifted to improve readability.

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