Document --module option for d8

- 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}

AUTHORS

@@ -128,6 +128,7 @@ Paul Lind <plind44@gmail.com>
 Qingyan Li <qingyan.liqy@alibaba-inc.com>
 Qiuyi Zhang <qiuyi.zqy@alibaba-inc.com>
 Rafal Krypa <rafal@krypa.net>
+Ray Glover <ray@rayglover.net>
 Refael Ackermann <refack@gmail.com>
 Rene Rebe <rene@exactcode.de>
 Rick Waldron <waldron.rick@gmail.com>

src/flags.cc

@@ -605,18 +605,12 @@ void FlagList::PrintHelp() {
   CpuFeatures::PrintFeatures();
 
   OFStream os(stdout);
-  os << "Usage:\n"
-        "  shell [options] -e string\n"
-        "    execute string in V8\n"
-        "  shell [options] file1 file2 ... filek\n"
-        "    run JavaScript scripts in file1, file2, ..., filek\n"
-        "  shell [options]\n"
-        "  shell [options] --shell [file1 file2 ... filek]\n"
-        "    run an interactive JavaScript shell\n"
-        "  d8 [options] file1 file2 ... filek\n"
-        "  d8 [options]\n"
-        "  d8 [options] --shell [file1 file2 ... filek]\n"
-        "    run the new debugging shell\n\n"
+  os << "Synopsis:\n"
+        "  shell [options] [--shell] [<file>...]\n"
+        "  d8 [options] [-e <string>] [--shell] [[--module] <file>...]\n\n"
+        "  -e        execute a string in V8\n"
+        "  --shell   run an interactive JavaScript shell\n"
+        "  --module  execute a file as a JavaScript module\n\n"
         "Options:\n";
 
   for (const Flag& f : flags) {