net/http: document ErrServerClosed

[vcabbage]

http.ErrServerClosed is currently documented only under http.Server.Serve(). It's purpose should also be documented where it's defined, like the other net/http errors.

@bradfitz Are documentation additions still valid for 1.8 or will this have to be 1.9?

[gopherbot]

CL https://golang.org/cl/36943 mentions this issue.