Add a note about FormatTime platform dependence (#908)

This has been another constant source of confusion since the C99 ones were added to the strftime documentation we link.

It'd be nice to have a consistent implementation inside SM in the future.

plugins/include/sourcemod.inc

@@ -335,6 +335,10 @@ native int GetTime(int bigStamp[2]={0,0});
  *
  * See this URL for valid parameters: 
  * http://cplusplus.com/reference/clibrary/ctime/strftime.html
+ * 
+ * Note that available parameters depends on support from your operating system.
+ * In particular, ones highlighted in yellow on that page are not currently
+ * available on Windows and should be avoided for portable plugins.
  *
  * @param buffer		Destination string buffer.
  * @param maxlength		Maximum length of output string buffer.