Improve protection message for edit

Right now we display the attached screenshot. I think we could improve it in 2 ways:

• Fix the English so that we don't use plural when it should be singular
• Provide more information and explain that it's not just a problem of breaking the extension but also that if the extension page is modified then when upgrading next this might cause a conflict that will need to be resolved (choosing either your version or the extension writer's and it might also need some manual merge as a consequence).