doc: add desc about gzip and http dump

Fix #3872

fs/fshttp/http.go

@@ -180,6 +180,13 @@ func NewTransportCustom(ci *fs.ConfigInfo, customize func(*http.Transport)) http
 	t.IdleConnTimeout = 60 * time.Second
 	t.ExpectContinueTimeout = ci.ExpectContinueTimeout
 
+	if ci.Dump&(fs.DumpHeaders|fs.DumpBodies|fs.DumpAuth|fs.DumpRequests|fs.DumpResponses) != 0 {
+		fs.Debugf(nil, "You have specified to dump information. Please be noted that the "+
+			"Accept-Encoding as shown may not be correct in the request and the response may not show "+
+			"Content-Encoding if the go standard libraries auto gzip encoding was in effect. In this case"+
+			" the body of the request will be gunzipped before showing it.")
+	}
+
 	// customize the transport if required
 	if customize != nil {
 		customize(t)