Document how comments can be used with text templating.

**Describe the problem/challenge you have**

I wanted to be able to have comments in file when using text templating, but couldn't find an example in the documentation. I therefore had to make an educated guess as to whether it was even possible and then work out how to do it.

**Describe the solution you'd like**

Add some documentation to:

* https://carvel.dev/ytt/docs/v0.40.0/ytt-text-templating/

The example should include use of the space trim character so that don't get extraneous blank lines in the output.

```
(@ load("@ytt:data", "data") -@)
(@= data.values.workshop.title @)
(@= "="*len(data.values.workshop.title) @)

(@# Comment -@)
(@# Comment -@)
(@# Comment -@)

(@= data.values.workshop.description @)
```

**Anything else you would like to add:**

Nope.

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