help: clarify contents of revlog index

The previous wording indicated that field at index 3 was the
size of the decompressed chunk, not the size of the full
revision text.

mercurial/help/internals/revlogs.txt

@@ -92,7 +92,8 b' 6-7 (2 bytes)'
 8-11 (4 bytes)
    Compressed length of revision data / chunk as stored in revlog.
 12-15 (4 bytes)
-   Uncompressed length of revision data / chunk.
+   Uncompressed length of revision data. This is the size of the full
+   revision data, not the size of the chunk post decompression.
 16-19 (4 bytes)
    Base or previous revision this revision's delta was produced against.
    -1 means this revision holds full text (as opposed to a delta).