Standardize some timestamp property descriptions

Author: cjdsellers

nautilus_trader/model/orders/base.pxd

@@ -133,7 +133,7 @@ cdef class Order:
     cdef readonly uint64_t ts_accepted
     """UNIX timestamp (nanoseconds) when the order was accepted or first filled (zero unless accepted or filled).\n\n:returns: `uint64_t`"""
     cdef readonly uint64_t ts_closed
-    """UNIX timestamp (nanoseconds) when the order closed (lifecycle completed).\n\n:returns: `uint64_t`"""
+    """UNIX timestamp (nanoseconds) when the order closed / lifecycle completed (zero unless closed).\n\n:returns: `uint64_t`"""
     cdef readonly uint64_t ts_last
     """UNIX timestamp (nanoseconds) when the last order event occurred.\n\n:returns: `uint64_t`"""
 

nautilus_trader/model/position.pxd

@@ -84,9 +84,9 @@ cdef class Position:
     cdef readonly uint64_t ts_last
     """UNIX timestamp (nanoseconds) when the last event occurred.\n\n:returns: `uint64_t`"""
     cdef readonly uint64_t ts_closed
-    """UNIX timestamp (nanoseconds) when the position was closed.\n\n:returns: `uint64_t`"""
+    """UNIX timestamp (nanoseconds) when the position was closed (zero unless closed).\n\n:returns: `uint64_t`"""
     cdef readonly uint64_t duration_ns
-    """The total open duration (nanoseconds).\n\n:returns: `uint64_t`"""
+    """The total open duration in nanoseconds (zero unless closed).\n\n:returns: `uint64_t`"""
     cdef readonly double avg_px_open
     """The average open price.\n\n:returns: `double`"""
     cdef readonly double avg_px_close