- I think d8's ability to load es6 modules is important enough to
document through the CLI
- I also tried to simplify the d8/shell CLI synopsis

This is my first patch; I can't run the automated test suite.

Change-Id: I6376542f57f11dd8ec53be9b53f3d17d46a86fed
Reviewed-on: https://chromium-review.googlesource.com/1056530Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53198}