The requirement that modified versions of the documents have the "same level of openness" was added after my first answer. If that is not a
requirement, and you just want to throw the documentation into the
world to be used however people want without attribution, you should
release the documents as public domain/creative commons zero.
If a "share alike" version is important, the only well known
documentation license I know of is the GNU FDL It's meant for
textbooks and manuals and is analogous to the GPL, so anyone
distributing the text needs to also distribute a human-editable
version of the text. So will anyone who redistributes more than 100
copies of the text.
Probably the easiest approach would be to require attribution and use
a CC-BY-SA license. Is it really so important to not be credited
for the documents?