net/http: idle timeout added in CL 121419 is undocumented and surprising

[bcmills]

CL 121419 changed net/http.Server to treat new connections as idle after 5 seconds, in order to unblock Shutdown with stalled connections (see #22682). That behavior has a regression test, but I find it surprising and I don't see it mentioned in the net/http documentation at all.

I would intuitively expect one of two other behaviors:

1. closeIdleConns could use the configured ReadHeaderTimeout for this purpose, as suggested in net/http: Server closeIdleConns does not close StateNew connections #22682 (comment).
2. A call to Shutdown could instead cause closeIdleConns to close all connections in StateNew, regardless of how long they have been in that state.

(attn @neild; CC @bradfitz @tombergan)