pcapng: clarify what the ISB capture start and end times are.

They're the times that capture started and ended on the interface in
question, which may not be the time that the capture, as a whole,
started, if, for example, the interface in question didn't exist when
the capture started, and capturing started when it was added.

draft-ietf-opsawg-pcapng.md

@@ -1877,11 +1877,11 @@ the following options are valid within this block:
 {: indent='8'}
 isb_starttime:
 : The isb_starttime
-  option specifies the time the capture started, consisting of two
-  unsigned 32-bit values, in the same format as defined for timestamps
-  in the Enhanced Packet Block ({{section_epb}}), using the 'if_tsresol'
-  and 'if_tsoffset' values from the Interface Description Block
-  specified by the Interface ID.
+  option specifies the time that traffic capture started on this
+  interface, consisting of two unsigned 32-bit values, in the same
+  format as defined for timestamps in the Enhanced Packet Block
+  ({{section_epb}}), using the 'if_tsresol' and 'if_tsoffset' values
+  from the Interface Description Block specified by the Interface ID.
 {: vspace='0'}
 
 Example: '96 c3 04 00 73 89 6a 65', in Little Endian, decodes
@@ -1891,11 +1891,11 @@ to 2012-06-29 06:17:00.834163 UTC.
 {: indent='8'}
 isb_endtime:
 : The isb_endtime
-  option specifies the time the capture ended, consisting of two
-  unsigned 32-bit values, in the same format as defined for timestamps
-  in the Enhanced Packet Block ({{section_epb}}), using the 'if_tsresol'
-  and 'if_tsoffset' values from the Interface Description Block
-  specified by the Interface ID.
+  option specifies the time that traffic capture ended on this
+  interface, consisting of two unsigned 32-bit values, in the same
+  format as defined for timestamps in the Enhanced Packet Block
+  ({{section_epb}}), using the 'if_tsresol' and 'if_tsoffset' values
+  from the Interface Description Block specified by the Interface ID.
 {: vspace='0'}
 
 Example: '97 c3 04 00 aa 47 ca 64', in Little Endian, decodes