Update MTProxy checker documentation and logic to clarify probing modes and improve error handling. Modify README and Docker documentation to reflect changes in the -probe flag behavior, including detailed descriptions of exit codes and timeout handling. Refactor connection handling to enhance robustness and align with Telethon's approach.

internal/checkresult/checkresult.go

@@ -16,9 +16,6 @@ func Classify(err error) (exitCode int, message string) {
 	if errors.Is(err, checker.ErrProxyClosed) {
 		return 3, err.Error()
 	}
-	if errors.Is(err, checker.ErrNoDataAfterHeader) {
-		return 1, err.Error()
-	}
 	if errors.Is(err, context.DeadlineExceeded) {
 		return 4, "timeout"
 	}