mb: Mention the user guide in the general help

Bug: 863028
Change-Id: Ie44f7eb962a76b3eb0fa58de4b6b32d2a11c3685
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1713709
Auto-Submit: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679811}

tools/mb/mb.py

@@ -108,7 +108,12 @@ class MetaBuildWrapper(object):
       subp.add_argument('-v', '--verbose', action='store_true',
                         help='verbose logging')
 
-    parser = argparse.ArgumentParser(prog='mb')
+    parser = argparse.ArgumentParser(
+      prog='mb', description='mb (meta-build) is a python wrapper around GN. '
+                             'See the user guide in '
+                             '//tools/mb/docs/user_guide.md for detailed usage '
+                             'instructions.')
+
     subps = parser.add_subparsers()
 
     subp = subps.add_parser('analyze',