Tiny fix: clarify in `run-external`'s signature that the external
command must be a string.
### Before
```
Signatures:
<any> | run-external <any> -> <any>
Parameters:
command <any>: external command to run
...args <any>: arguments for external command
```
### After
```
Signatures:
<any> | run-external <string> -> <any>
Parameters:
command <string>: external command to run
...args <any>: arguments for external command
```
### Notes
I was hoping to change more `any`s to more specific types, but alas I
think we can only change `command` right now. The input can be any type
and it gets rendered to a string before being passed to the external.
The args can be any value type and they get converted to strings. The
output can be either binary or a string.
7413ef2824