add descriptions to issue templates

.github/ISSUE_TEMPLATE/10_bug_report.yml

 name: Bug report
 labels: [bug]
+description: Something isn't working right.
 body:
   - type: textarea
     id: description

.github/ISSUE_TEMPLATE/20_feature_request.yml

 name: Feature request
 labels: ['feature request']
+description: Request a new feature.
 body:
   - type: textarea
     id: problem

.github/ISSUE_TEMPLATE/30_model_request.yml

 name: Model request
 labels: ['model request']
+description: Request a new model.
 body:
   - type: textarea
     id: problem