Tell people not to propose hypothetically useful APIs

Author: joshtriplett

.github/ISSUE_TEMPLATE/api-change-proposal.md

@@ -14,7 +14,7 @@ assignees: ""
 
 ## Motivating examples or use cases
 
-<!-- Next add any motivating examples. Examples should ideally be real world examples, or minimized versions of the real world example in scenarios where the motivating code is not open source. -->
+<!-- Next add any motivating examples. Examples should ideally be real world examples, or minimized versions of the real world example in scenarios where the motivating code is not open source. Don't propose changes you think might *hypothetically* be useful; real use cases help make sure we have the right design. -->
 
 ## Solution sketch