docs: Clarify on-disk order and size of log entry fields (#6357)

docs: clarify on-disk order and size of log entry fields

The order of the fields of a log entry on disk is CRC32 first, the docs had the size first.

I tried to make this list similar to the HashIndex struct description.

docs/internals/data-structures.rst

@@ -94,13 +94,13 @@ this value in a non-empty repository, you may also need to relocate the segment
 files manually.
 
 A segment starts with a magic number (``BORG_SEG`` as an eight byte ASCII string),
-followed by a number of log entries. Each log entry consists of:
+followed by a number of log entries. Each log entry consists of: (in this order)
 
-* size of the entry
-* CRC32 of the entire entry (for a PUT this includes the data)
-* entry tag: PUT, DELETE or COMMIT
-* PUT and DELETE follow this with the 32 byte key
-* PUT follow the key with the data
+* First, unsigned 32-bit number, the CRC32 of the entire entry (for a PUT including the DATA) excluding the CRC32 field
+* Second, unsigned 32-bit size of the entry (including the whole header)
+* Third, unsigned 8-bit entry tag: PUT(1), DELETE(2) or COMMIT(3)
+* Fourth, on PUT or DELETE, 32 byte key
+* Fifth, PUT only, (size - 41) bytes of data (length = size - sizeof(CRC32) - sizeof(size) - sizeof(entry tag) - sizeof(key))
 
 Those files are strictly append-only and modified only once.