emerge clean has been deprecated in favor of emerge --clean. Such a typo appears in the manpage. See the patch for the detail.
Created attachment 104664 [details, diff]
Correction to the manpage in SVN - trunk
Thanks, in svn r5369
Checked. Thanks for your attention!!!
careful, not fixed until it's released ;)
This has been released in 2.1.2_rc4-r1.