If I release a program specification  under the AGPL, and the program itself under the AGPL, the two are obviously compatible: I can develop them at the same time, copy text (for example, method headers) between them freely, "derive" methods from the spec requirements, back-derive requirements from implementation (it happens, ok ;). But, the specification would not be compatible with BY-SA works (say, Wikipedia or Stack Exchange).
If I release the specification under BY-SA instead, it is still compatible in one direction (from spec to software), because BY-SA is one-way compatible with the GPL , and the GPL is compatible with the AGPL.
But, if I release the specification under BY-SA (including releasing it before the software), am I "losing" any of the copyleft strength of the AGPL. i.e. enabling someone to create a non-copyleft work where they couldn't before?
(Looking at it another way, if I release just an AGPL specification, am I guaranteeing that any implementation must be AGPL? Or is anyone free to develop the "ideas" in the spec under any license they want?)
 To clarify, a specification is not end-user documentation (a manual), it is a (sometimes very detailed) outline and plan for what the software must do.
The answer obviously rests on the judgment of to what extent the code is a derivative work of the specification. I will obviously accept an answer of "it depends", "not tested in law", etc, if that is the most accurate answer possible :D
 It appears I am dreaming and this is underway, but not yet finalised. Let's assume for the sake of argument we are a few months in the future and it's true.