net/http: fix Server.IdleTimeout and Server.ReadHeaderTimeout docs

CL 46434 changed the doc for Server.IdleTimeout to include
falling back to Server.ReadHeaderTimeout if both
Server.IdleTimeout and Server.ReadTimeout are zero.
However, we explicitly set the ReadDeadlines firstly based
off Server.IdleTimeout or Server.ReadTimeout before attempting
to read the next request, thus the current doc is incorrect.

This CL reverts CL 46434 and also updates the doc for
Server.ReadHeaderTimeout to documenting falling back
to Server.ReadTimeout, if the former is zero, otherwise
there is no timeout.

Fixes #32053

Change-Id: I43dd0252d1bcee6c29a8529abd84c84a49b2fba9
GitHub-Last-Rev: e1cdb599772476e0b52d9d2374c3ea3893844eb4
GitHub-Pull-Request: golang/go#32164
Reviewed-on: https://go-review.googlesource.com/c/go/+/178337


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>