docs: add usage examples section to api spec template (#321)

wg-api/spec-documents/api-spec-template.md

@@ -12,5 +12,8 @@
 ## API Design
 <!--- Discuss any background and motivation not already in the summary above. Give implementation details. What options are under consideration, and what is your preferred solution or approach? --->
 
+### Usage examples
+<!-- Give concrete examples of how this API will be used. -->
+
 ## Rollout Plan
 <!--- What branches do you ultimately want this API to live in? If this change would require a minor release, please justify the reason for that request. -->