Documentation change only: clarify uPortQueueGetFree() is optional.
Add a note to the uPortQueueGetFree() function documentation to indicate that it is not a requirement that the function is implemented, it can return U_ERROR_COMMON_NOT_IMPLEMENTED and the operation of ubxlib will not be adversely affects.  This note was already present on uPortEventQueueGetFree() but was missed here.
@@ -223,6 +223,9 @@ int32_t uPortQueueTryReceive(const uPortQueueHandle_t queueHandle,
int32_t waitMs, void *pEventData);
/** Get the number of free spaces in the given queue.
+ * It is NOT a requirement that this API is implemented:
+ * where it is not implemented U_ERROR_COMMON_NOT_IMPLEMENTED
+ * should be returned.
*
* @param queueHandle the handle of the queue.
* @return on success the number of spaces available,
