Update new-ontology.yml instructions to better conform to principle 6

In order to promote the importance of term descriptions, the instructions have been revised to remove the clause "while not necessary". At the same time, the necessity of descriptions for top-level terms is now emphasized. These changes bring the new ontology request form in better alignment with principle 6.

.github/ISSUE_TEMPLATE/new-ontology.yml

@@ -268,7 +268,7 @@ body:
           required: false
         - label: My term labels are in English and conform to the OBO Foundry [Naming Conventions](http://obofoundry.org/http://obofoundry.org/principles/fp-012-naming-conventions.html)
           required: false
-        - label: I understand that term definitions, while not mandatory, are key to understanding the intentions of a term especially when the ontology is used in curation. I made sure that a reasonable majority of terms in my ontology have definitions, in English, using the [IAO:0000115](https://ontobee.org/ontology/IAO?iri=http://purl.obolibrary.org/obo/IAO_0000115) property.
+        - label: I understand that term definitions are key to understanding the intentions of a term, especially when the ontology is used in curation. I made sure that a reasonable majority of terms in my ontology--and all top level terms--have definitions, in English, using the [IAO:0000115](https://ontobee.org/ontology/IAO?iri=http://purl.obolibrary.org/obo/IAO_0000115) property.
           required: false
         - label: For every term in my ontology, I checked whether another OBO Foundry ontology has one with the same meaning. If so, I re-used that term directly (not by cross-reference, by directly using the IRI).
           required: false