Document what happens if you pass an invalid error code

to SystemError or WindowsError

format.h

@@ -1427,6 +1427,8 @@ class SystemError : public internal::RuntimeError {
    where *<message>* is the formatted message and *<system-message>* is
    the system message corresponding to the error code.
    *error_code* is a system error code as given by ``errno``.
+   If *error_code* is not a valid error code such as -1, the system message
+   may look like "Unknown error -1" and is platform-dependent.
    
    **Example**::
 
@@ -2109,9 +2111,7 @@ void report_system_error(int error_code, StringRef message) FMT_NOEXCEPT(true);
 
 #ifdef _WIN32
 
-/**
- A Windows error.
-*/
+/** A Windows error. */
 class WindowsError : public SystemError {
  private:
   void init(int error_code, StringRef format_str, ArgList args);
@@ -2127,7 +2127,9 @@ class WindowsError : public SystemError {
    where *<message>* is the formatted message and *<system-message>* is the system
    message corresponding to the error code.
    *error_code* is a Windows error code as given by ``GetLastError``.
-   
+   If *error_code* is not a valid error code such as -1, the system message
+   will look like "error -1".
+
    **Example**::
 
      // This throws a WindowsError with the description