Aidan Kehoe writes: 
 
That means usually that the DOC file the binary thinks it should use
doesn’t correspond to its version. We should really version the DOC
files in the same as we version the .dmp files, and avoid over-writing
them. 

Stephen Turnbull responds:

I agree that sounds plausible, but somebody on emacs-devel was arguing 
against versioning of DOC files any finer than the version of Emacs 
(Emacs uses a build number instead of our "corrupted timestamp").

I wonder if it's worth providing a fallback to whatever DOC file we can
find.