FWIW, I used to work as a technical writer. To treat this issue with the seriousness it deserves, I suggest hiring a copy editor with a technical background, either full time or on contract, with an initial brief to standardize a house style on which everyone can agree. Older people are a great source of potential hires because many of them went to school when grammar used to be taught. As an aside, I think you're doing the right thing by getting developers to document their own work even if their documents need rewriting by someone else, because doing so compels them to think clearly.