emerge(1) lacks an explanation of USE flags in parentheses.
Sure it does: () circumfix = forced, masked, or removed
I was looking in the 'OUTPUT' section. This should maybe be mentioned there, too, not only under the description for '-v'.
Created attachment 118609 [details, diff] add a reference the --verbose option documentation in the OUTPUT section This little patch is in svn r6504.
This has been released in 2.1.2.8.