Error messages should guide users toward the "parent" or nested resource syntax

**Is your feature request related to a problem? Please describe.**
When declaring a nested resource type, we show an error message about wrong number of slashes in the resource name:

**Describe the solution you'd like**
The message is technically correct, but a better suggestion would be to point to the `parent` syntax or to the nested resource syntax.

Start by locating the diagnostic that reports the wrong number of slashes in a nested resource name. Review the issue's screenshot and compare the current wording with the requested guidance toward `parent` syntax or nested resource syntax; done means the error message clearly points users to one of those forms and its diagnostic coverage passes.

Written by the indexing model from the issue text.