plots: point template help info to manual

[pared]

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here. If the CLI API is changed, I have updated tab completion scripts.

• ❌ I will check DeepSource, CodeClimate, and other sanity checks below. (We consider them recommendatory and don't expect everything to be addressed. Please fix things that actually improve code or fix bugs.)

Thank you for the contribution - we'll try to review it as soon as possible. 🙏
Fixes #3755

[jorgeorpinel]

I found this https://github.com/vega/schema are templates basically this? Vega schema files. If so, I suggest

Suggested change

	"Vega specification file to inject with the data. "
	"Vega JSON schema file to inject with the data. "

and will update the docs to link to this repo for reference.

[pared]

Hmm actually template is an Vega JSON schema with DVC placeholders (anchors) that will be filled with proper values after using dvc plots. Also, template does not have to be JSON at all,
that is bad on my side. If user wants it can be HTML as well, the only thing that makes it template is existence of DVC_METRIC_DATA anchor inside.

Also this might change depending on discussion under #3818

So I think that as of now, the best description would be to leave it as is (File to be injected with data. See {link}

[jorgeorpinel]

OK np let's just come up with a general description here and we'll have to revisit the docs to explain this a little better. The anchors are listed but it's not clear that templates are a combo of Vega schema and DVC anchors + possibly HTML. TBH I still don't totally get it haha. But how about

Suggested change

	"Vega specification file to inject with the data. "
	"Special JSON or HTML schema to inject with the data. "

?

Also this might change depending on discussion under #3818

No worries. We can revisit this and docs if/when that happens.

[jorgeorpinel]

p.s. added checkbox to improve plot template desc. to #1175.

[jorgeorpinel]

Forgot a little detail but yes, looks good. Thanks!