Re-expose the --bootstrap option, just with better help.

Rather than requiring toolchain/folks screwing w/ sdk affecting
changes to hunt down one of us, then us telling them about the
hidden option, instead just document it appropriately.

BUG=chromium-os:33737
TEST=cros_sdk --help

Change-Id: I1c0a0bf2af1e1dab5b19e5159430e9a721413502
Reviewed-on: https://gerrit.chromium.org/gerrit/31074
Commit-Ready: Brian Harring <ferringb@chromium.org>
Reviewed-by: Brian Harring <ferringb@chromium.org>
Tested-by: Brian Harring <ferringb@chromium.org>
diff --git a/scripts/cros_sdk.py b/scripts/cros_sdk.py
index 8745da7..3f484d1 100644
--- a/scripts/cros_sdk.py
+++ b/scripts/cros_sdk.py
@@ -273,7 +273,12 @@
   # Actions:
   parser.add_option('--bootstrap',
                     action='store_true', dest='bootstrap', default=False,
-                    help=optparse.SUPPRESS_HELP)
+                    help=('Build everything from scratch, including the sdk.  '
+                          'Use this only if you need to validate a change '
+                          'that affects SDK creation itself (toolchain and '
+                          'build are typically the only folk who need this).  '
+                          'Note this will quite heavily slow down the build.  '
+                          'Finally, this option implies --enter.'))
   parser.add_option('--delete',
                     action='store_true', dest='delete', default=False,
                     help=('Delete the current SDK chroot'))