Automating the first 50-80%? Try some very simple natural language processing, with a human being acting as a sanity check. If the PHP documentation for a function is "upper case" and the Python documentation for something also is "upper case," then the two might be similar...
In practice, I'm not sure what it looks like. I wonder if something like "minimum number of edits" would work? Probably not, I guess since that's more like a "diff" and the docs are not likely to be similar in that way.
Perhaps just "the function who's documentation uses the most same words", stripping common words? Perhaps that could be improved a bit via synonyms too.
Automating the first 50-80%? Try some very simple natural language processing, with a human being acting as a sanity check. If the PHP documentation for a function is "upper case" and the Python documentation for something also is "upper case," then the two might be similar...