I didn't even find it in the EAPI documentation.
Steps to Reproduce:
That's because it doesn't exist. There's package.unmask, and use.mask, but that's it. Read man portage. Whatever you're seeing, either it doesn't actually exist, or it's in an unstable Portage version, or else it's something that only belongs in a profile and is not meant to be user-set.
Portage team: do let us know if it's something that actually exists and if you want end-user documentation for it. Otherwise, closing as INVALID.
There is no use.unmask. We use negative settings to undo use.mask settings.
Then how do you officially unmask the gallium USE in mesa from the x11 overlay?
echo -gallium >> /etc/portage/profile/use.mask
So use.mask doesn't have a documentation.
There's some documentation in the 'portage' man page, but it lacks examples.
(In reply to comment #3)
> Then how do you officially unmask the gallium USE in mesa from the x11 overlay?
Bugzilla is not a support venue. Please take your questions to the Gentoo Forums (http://forums.gentoo.org), in the Unsupported Software area, which is where all questions regarding overlays belong.
I think it should also be in make.conf man.
Oh... no, on second thought, it's ok there itself.
I've added some examples here: