Update the help text of bisect-builds.py tool.

tools/bisect-builds.py

Index: tools/bisect-builds.py
diff --git a/tools/bisect-builds.py b/tools/bisect-builds.py
index e601e4792094263d96eb11369110f7e2e882ce69..fd6810cd2cb5626c345b7983b433300c7fd7168b 100755
--- a/tools/bisect-builds.py
+++ b/tools/bisect-builds.py
@@ -587,7 +587,11 @@ def GetChromiumRevision(url):
 
 def main():
   usage = ('%prog [options] [-- chromium-options]\n'
-           'Perform binary search on the snapshot builds.\n'
+           'Perform binary search on the snapshot builds to find a minimal '
+           'range of revisions where a behavior change happened. The '
+           'behaviors are described as "good" and "bad". '
+           'It is NOT assumed that the behavior of the later revision is '
+           'the bad one.\n'
            '\n'
            'Tip: add "-- --no-first-run" to bypass the first run prompts.')
   parser = optparse.OptionParser(usage=usage)
@@ -603,9 +607,12 @@ def main():
                     'Chrome builds (internal only) instead of ' +
                     'Chromium archives.')
   parser.add_option('-b', '--bad', type = 'str',
-                    help = 'The bad revision to bisect to. Default is HEAD.')
+                    help = 'A bad revision to start bisection. ' +
+                    'May be earlier or later than the good revision. ' +
+                    'Default is HEAD.')
   parser.add_option('-g', '--good', type = 'str',
-                    help = 'The last known good revision to bisect from. ' +
+                    help = 'A good revision to start bisection. ' +
+                    'May be earlier or later than the bad revision. ' +
                     'Default is 0.')
   parser.add_option('-p', '--profile', '--user-data-dir', type = 'str',
                     help = 'Profile to use; this will not reset every run. ' +