Documentation license
I have concerns about both options:
- OSD-compliant would mean that documentation would need to be under a license that fulfils all ten OSD criteria, and many of those are quite software-specific. This could be tricky, there is a reason why OSI hasn’t approved (m)any non-software licenses thus far.
- as for OSD-compatible, it seems a bit ambiguous what that even means. So many proprietary licenses are compatible with so many (non-copyleft) OSD-compliant licenses, that I fear this means almost nothing. (Well, not exactly nothing, as it does mean the documentation license cannot remove the FOSS’ness of the code license, at least.)
Open Knowledge Foundation (OKFN) has their own “Open Definition”, which is perhaps the best drafted definition of open data and open content I’ve seen so far:
(There is also the Definition of Free Cultural Works, based on the 4 freedoms, which used to have quite some traction back in the day, but I haven’t followed its development in ages, to see how it fares nowadays. It seems the definition has not changed much in almost a decade.)
As such, I propose to adopt The Open Definition from OKFN for non-code stuff.
Code license 
A further complication is whether (for code) we’d also consider licenses that are tagged as:
- non-reusable
- redundant
- superseded
- voluntarily retired
- …
These are technically OSI-approved, just that OSI suggest not to use them.
The new tagging/classification of licenses is a great thing, but it does have consequences, and allows for more finer-tuned policies.