Improve BarrierNode documentation around deletes

The documentation for BarrierNode shows the following example:

```
stream
|from()
.measurement('cpu')
|barrier()
.idle(5s)
.delete()
...
```

When using this construction of a barrier as shown in the documentation with `idle()` followed by `delete()` we have been seeing the following error (Kapacitor 1.5.2):

```
invalid TICKscript: line <#> char <#>: too many arguments to set property Delete on *pipeline.BarrierNode
```

We've tried various constructions with a different order and even passing a boolean to `.delete()`. Nothing seems to satisfy the syntax. Is the example in the documentation wrong? Or, are we just flat out missing something assumed by the example? Or, is this a bug in the parser for this node?

Start with the BarrierNode documentation example and the parser handling of the .idle() and .delete() calls. Determine whether the shown syntax is invalid or whether the parser rejects valid syntax; document the correct construction or clarify the parser behavior, with the example producing a valid TICKscript.

Written by the indexing model from the issue text.