text: Title: missing detail in grid documentation
  Body: in the docs we have an example that looks like this:
  
  ```yaml
  tasks:
    - source: random-forest.py
      name: random-forest-
      product: 'n_estimators=[[n_estimators]]/criterion=[[criterion]].html'
      grid:
          n_estimators: [5, 10, 20]
          criterion: [gini, entropy]
  
  ```
  
  [link](https://docs.ploomber.io/en/latest/api/spec.html#tasks-grid)
  
  the `[[placeholders]]`` are replaced at runtime by the parameter values, however, to ensure each file has a different name we also append a number at the end; but this isn't documented.
  
  right below the snippet that I put above, we should add a little note saying that we number the filenames to prevent conflicting paths.
  
  