lib: add comment to script eval _tickCallback

PR'ing this so it doesn't get forgotten, see https://github.com/nodejs/node/pull/11958#discussion_r107184672

Add a comment to match lib/module.js, missed in #11958.

Refs: https://github.com/nodejs/node/pull/11958

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows [commit guidelines][]

Affected core subsystem(s)

lib

cc/ @Fishrock123 @bnoordhuis @jasnell