Improve feature proposal template for improved product thinking

.gitlab/issue_templates/Feature proposal.md

 ### Problem to solve
 
+<!--- What problem do we solve? -->
+
+### Target audience
+
+<!--- For whom are we doing this? Include either a persona from https://design.gitlab.com/#/getting-started/personas or define a specific company role. e.a. "Release Manager" or "Security Analyst" -->
+
 ### Further details
 
-(Include use cases, benefits, and/or goals)
+<!--- Include use cases, benefits, and/or goals (contributes to our vision?) -->
 
 ### Proposal
 
+<!--- How are we going to solve the problem? -->
+
 ### What does success look like, and how can we measure that?
 
-(If no way to measure success, link to an issue that will implement a way to measure this)
+<!--- If no way to measure success, link to an issue that will implement a way to measure this -->
 
 ### Links / references