Newer versions of this package are in the tree as dev-php/phpDocumentor. Nothing needs it under the old name.
commit cec0ff0a58a083f9b191a6264bad4623cd784c2f Author: Michael Orlitzky <mjo@gentoo.org> Date: Tue Mar 14 11:30:56 2017 -0400 dev-php/PEAR-PhpDocumentor: remove masked package. Gentoo-Bug: 609138