Improve head comment on JS (#12548)
Improve head comment on JS (#12548)

* Improve comment on JSGen, replace 12+ concatenations by 1 multiline string

* Peer review feedbacks https://github.com/nim-lang/Nim/pull/12548#discussion_r340051147

dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim,dom96/Nim