Update common function body comment
Update common function body comment

- This commit provides a canonical way of describing
  that the function body contains code of some sort.
- The convention chosen was '// ...'.
- This change is persisted throughout the readme file
  where appropriate

LodoSoftware/javascript-style-guide,pokelondon/javascript,webcoding/javascript-style-guide,xalexec/javascript,getjll/JavaScript-Style-Guide,appcom-interactive/javascript,bl00mber/javascript,practicefusion/javascript,airbnb/javascript,allanfish/javascript,cloudability/javascript-style-guide,addshoppers/javascript,whichdigital/javascript-coding-standards,ljharb/javascript,LodoSoftware/javascript-style-guide