Minor improvements to wxSysErrorMsgStr() documentation

Link to the new function, instead of wxSysErrorMsg(), from the other functions
documentation.

Also mention that this function is new since 3.1.0.

interface/wx/log.h

@@ -1215,7 +1215,7 @@ void wxSafeShowMessage(const wxString& title, const wxString& text);
     Returns the error code from the last system call. This function uses
     @c errno on Unix platforms and @c GetLastError under Win32.
 
-    @see wxSysErrorMsg(), wxLogSysError()
+    @see wxSysErrorMsgStr(), wxLogSysError()
 
     @header{wx/log.h}
 */
@@ -1229,6 +1229,8 @@ unsigned long wxSysErrorCode();
     Use this function instead of wxSysErrorMsg(), as the latter one is not
     thread-safe.
 
+    @since 3.1.0
+
     @see wxSysErrorCode(), wxLogSysError()
 
     @header{wx/log.h}
@@ -1457,7 +1459,7 @@ void wxVLogStatus(const char* formatString, va_list argPtr);
     form of this function takes the error code explicitly as the first
     argument.
 
-    @see wxSysErrorCode(), wxSysErrorMsg()
+    @see wxSysErrorCode(), wxSysErrorMsgStr()
 
     @header{wx/log.h}
 */