proposal: net/http: decide and document accepted HTTP methods in FileServer

[pascaldekloe]

Now that the fix got reversed, the documentation should at least inform/warn about it's broken by default.

[mknyszek]

CC @neild

[rsc]

I would still support a change to reject methods other than GET, HEAD, POST. This is the kind of protocol compliance change that would be good to use a GODEBUG to control and roll out.