clarify language around empty cptures segments

and that every sample in a dataset exists in exactly one captures
segment.

sigmf-spec.md

@@ -475,10 +475,15 @@ field to associate up to the relevant `sigmf-collection` file.
 
 #### Captures Array
 
-The `captures` value is an array of `capture segment` Objects that describe the
+The `captures` field is an array of `capture segment` Objects that describe the
 parameters of the signal capture. It MUST be sorted by the value of each
 capture segment's `core:sample_start` key, ascending.
 
+All samples in a dataset are contained in exactly one capture segment. While
+there SHOULD be at least one segment defined, there are no items in the captures
+array it is implied that a single capture with the `core:sample_start` field
+equal to zero; no other metadata is implied.
+
 ##### Capture Segment Objects
 
 Capture Segment Objects are composed of key/value pairs, and each Segment describes