clarify storage of timestamps and refs in the index

Documentation/format/index.md

@@ -80,9 +80,9 @@ The file offset to the beginning of a series serves as the series' ID in all sub
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 Every series entry first holds its number of labels, followed by tuples of symbol table references that contain the label name and value. The label pairs are lexicographically sorted.  
-After the labels, the number of indexed chunks is encoded, followed by a sequence of metadata entries containing the chunks minimum and maximum timestamp and a reference to its position in the chunk file. Holding the time range data in the index allows dropping chunks irrelevant to queried time ranges without accessing them directly.
+After the labels, the number of indexed chunks is encoded, followed by a sequence of metadata entries containing the chunks minimum (`mint`) and maximum (`maxt`) timestamp and a reference to its position in the chunk file. The `mint` is the time of the first sample and `maxt` is the time of the last sample in the chunk. Holding the time range data in the index allows dropping chunks irrelevant to queried time ranges without accessing them directly.
 
-mint of the first chunk is stored and the mint and maxt are encoded as deltas to the previous time. Similarly, the first delta is stored and the next ref is stored as a delta to the previous.
+`mint` of the first chunk is stored, it's `maxt` is stored as a delta and the `mint` and `maxt` are encoded as deltas to the previous time for subsequent chunks. Similarly, the reference of the first chunk is stored and the next ref is stored as a delta to the previous one.
 
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