cleanup-workdir: update help text

Update the help text to tell user that the files and dirs under WORKDIR
which are not created by Yocto will be deleted.

(From OE-Core rev: dcd2ebc5d63965a7ad6e714406149f63ffb4a704)

Signed-off-by: Kang Kai <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/cleanup-workdir

@@ -65,8 +65,11 @@ def main():
     parser = optparse.OptionParser(
         usage = """%prog
 
-Remove the obsolete packages' build directories in WORKDIR.
-This script must be ran under BUILDDIR after source file \"oe-init-build-env\".""")
+%prog removes the obsolete packages' build directories in WORKDIR.
+This script must be ran under BUILDDIR after source file \"oe-init-build-env\".
+
+Any file or directory under WORKDIR which is not created by Yocto
+will be deleted. Be CAUTIOUS.""")
 
     options, args = parser.parse_args(sys.argv)