Link content type help message is misleading

[sneridagh]

https://github.com/plone/plone.app.contenttypes/blob/master/plone/app/contenttypes/schema/link.xml#L8

As explained in:
plone/plone.restapi#1197

We should improve it to specify a better wording for the variable interpolation use case/feature.

[yurj]

It works, you've to set it in external. So that message is valid only for external links.