docs(stdio): put --log before the task name in the example (#874)

## Motivation

`docs/stdio.md` shows `vp run build --log=<mode>`. `vp run` stops
reading its own flags at the task name, so that command passes
`--log=<mode>` to the task and keeps the default output mode:

```
$ vt run build --log=labeled
$ vtt print built --log=labeled ⊘ cache disabled
built --log=labeled
```

## Changes

- Show `vp run --log=<mode> build`.
- Say that arguments after the task name go to the task.

Author: Dunqing

docs/stdio.md

@@ -5,9 +5,11 @@ How stdin, stdout, and stderr are connected to task processes, controlled by the
 ## The `--log` Flag
 
 ```
-vp run build --log=<mode>
+vp run --log=<mode> build
 ```
 
+The flag goes before the task name. Anything after the task name is passed to the task, so `vp run build --log=grouped` runs the task's command with `--log=grouped` appended.
+
 | Mode                        | Description                                                                   |
 | --------------------------- | ----------------------------------------------------------------------------- |
 | `interleaved` **(default)** | Output streams directly to the terminal as tasks produce it.                  |