Update docs/capture.rst

Co-authored-by: Tom C (DLS) <[email protected]>

docs/capture.rst

@@ -138,9 +138,11 @@ At the beginning of each experiment the following information is sent:
 
 ================= ==============================================================
 arm_time          System timestamp when ARM command was sent
-start_time        Timestamp of when PCAP became both armed and enabled
-hw_time_offset_ns Offset in ns from hardware timestamp to captured system time,
-                  only present when hardware time source is used.
+start_time        Timestamp of when PCAP became both armed and enabled.
+                  Uses hardware provided timestamp (e.g. from event receiver) if available, falling back to the system timestamp.
+hw_time_offset_ns Offset in ns from hardware timestamp to system time at the start of the experiment.
+                  Only present when hardware time source is used.
+                  Used to check that hardware and system times have not drifted too far apart.
 missed            Number of samples missed by late data port connection.
 process           Data processing option: Scaled, Unscaled, or Raw.
 format            Data delivery formatting: ASCII, Base64, Framed, or Unframed.