I can’t think of any grammar rules that are more applicable to documentation than to other writing. Thus, I think giving an overview of grammar rules would be out-of-scope for this site, and shouldn’t be added. However, it might be in scope for this site to have an appendix with a list of links about writing in general, and such a grammar site would deserve a spot.
> I believe that most readers of that site will be non-native speakers
I don’t see why this site would get a higher proportion of non-native speakers than any other site. Learning to understand a language and learning to write well in it are mostly separate things, and I think native speakers will understand that, so they won’t avoid using this site just because it’s about writing in English. Or are you saying that there are more non-native-speaker users in general who browse English programming sites than native-speaker users?
> I believe that most readers of that site will be non-native speakers
I don’t see why this site would get a higher proportion of non-native speakers than any other site. Learning to understand a language and learning to write well in it are mostly separate things, and I think native speakers will understand that, so they won’t avoid using this site just because it’s about writing in English. Or are you saying that there are more non-native-speaker users in general who browse English programming sites than native-speaker users?