clean up the docs of some modules under lib/js

[ringabout]

• clean up the docs
• fixees misc cleanups timotheecour/Nim#64 (comment)

[ringabout]

see nim-lang/fusion#57 (comment)

[timotheecour]

LGTM; in future work

when not defined(js):
  {.error: "This module only works on the JavaScript platform".}

would need to use fatal instead, see rationale here: timotheecour#64 (comment)

EDIT: some tests fail that are related to this PR ...

not correct now

[timotheecour]

not correct now

proposed fix: simply remove lib/js/jscore.nim from withoutIndex in kochdocs.nim
=> this will then correctly run docs for lib/js/jscore.nim

(another fix would've been this:

   for d in items(withoutIndex):
-    commands[i] = nim & " doc $# --git.url:$# -o:$# $#" %
-      [nimArgs, gitUrl,
+    let extra = if isJsOnly(d): "--backend:js" else: ""
+    commands[i] = nim & " doc $# $# --git.url:$# -o:$# $#" %
+      [extra, nimArgs, gitUrl,
       destPath / changeFileExt(splitFile(d).name, "html"), d]
     i.inc

but the 1st proposed fix is better/simpler)

[ringabout]

I see, thanks, you are always very helpful!(I was testing it too)

[timotheecour]

LGTM again (if passes CI)