doc: eliminate commit-message skew between Contribution Guide and wiki

After @rasky's updates, https://tip.golang.org/doc/contribute.html#commit_messages has a nice, clear guide to Go commit messages. https://github.com/golang/go/wiki/CommitMessage has a very similar guide.

Unfortunately, they have a bit of skew: for example, the wiki mentions a 76-column limit, whereas contribute.html does not.

We should eliminate that skew: either both documents should list the full details, or one should provide a more concise summary and link to the other for details.