[ghc-steering-committee] Attempt at clarifying the template language

Dear all, I just wrote a short PR[1] which attempts to clarify the language in the template. Trying to dispel some confusion that seems to frequently arise as to what our expectations are, for proposals. Basically, I tried to announce more clearly the things I believe we find ourselves in need to say to proposal authors often about the purpose of sections. I probably forgot some. So bring in your shepperd experience. [1]: https://github.com/ghc-proposals/ghc-proposals/pull/358