Fix module comments in IDL compiler

Remove the "FIXME: Not currently used in build" (since now are)
and link to design docs.

BUG=345503
TBR=haraken

Review URL: https://codereview.chromium.org/190553006

git-svn-id: svn://svn.chromium.org/blink/trunk@168823 bbb929c8-8fbe-4397-9dbb-9b2b20218538