Doxygen: document how the SCardListReaders() reader list is formatted

Thanks to Olivier LORENTE [ST] for the bug report
"Multi-string with list of readers #233"
Closes: #233

Author: LudovicRousseau

src/winscard_clnt.c

@@ -2850,6 +2850,10 @@ LONG SCardTransmit(SCARDHANDLE hCard, const SCARD_IO_REQUEST *pioSendPci,
  * Encoding:
  * The reader names and group names are encoded using UTF-8.
  *
+ * The reader names is a multi-string and separated by a null character
+ * ('\0') and ended by a double null character like
+ * "Reader foo 00 00\0Reader bar 01 00\0\0".
+ *
  * @ingroup API
  * @param[in] hContext Connection context to the PC/SC Resource Manager.
  * @param[in] mszGroups List of groups to list readers (not used).