Currently, all of the Qt5 packages have their latest (unstable) versions as 5.6.0, however dev-qt/qt-docs is currently available only as 5.5.1_p0 and 5.6.1_p0, neither of which will be picked up by the latest (unstable) version of dev-qt/assistant:5, which again is at 5.6.0. dev-qt/assistant:5 will only pick up documentation for the same version qt-docs, which for unstable systems is currently impossible. There is no Qt 5.6.1 collection of packages to upgrade to, and there is no qt-docs-5.6.0_p0 to downgrade to for the assistant to work out-of-the-box.
I made a local ebuild for qt-docs-5.6.0_p1, and it seems to have done the trick (no attachement, since copying the ebuild to a different filename is enough). Note that there is no _p0 version for 5.6.0, except for one labeled "rc". My guess is that this bug was the result of a simple transposing typo in the filename; 1_p0 instead of the correct 0_p1.
You can easily point Qt Assistant to any versions of the documentation, there is no dependency between assistant and qt-docs. In fact, IIRC, you can even have multiple doc versions registered with Qt Assistant at the same time (if qt-docs were slotted). So I'm not going to restore qt-docs-5.6.0_p1. Use 5.6.1_p0 docs, they should be strictly better. Besides, dev-qt/assistant-5.6.1 is in the tree now.
