docs: add multi chunks layout in chunks readme file (#567)

docs/format/chunks.md

@@ -1,15 +1,31 @@
 # Chunks Disk Format
 
-The following describes the format of a single chunks file, which is created in the `chunks/` directory of a block. The maximum size per segment file is 512MiB.
+The following describes the format of a chunks file,
+which is created in the `chunks/` directory of a block.
+The maximum size per segment file is 512MiB.
 
-Chunks in the files are referenced from the index by uint64 composed of in-file offset (lower 4 bytes) and segment sequence number (upper 4 bytes).
+Chunks in the files are referenced from the index by uint64 composed of
+in-file offset (lower 4 bytes) and segment sequence number (upper 4 bytes).
 
 ```
-┌────────────────────────────────────────┬──────────────────────┐
-│ magic(0x85BD40DD) <4 byte>             │ version(1) <1 byte>  │
-├────────────────────────────────────────┴──────────────────────┤
-│ ┌───────────────┬───────────────────┬──────┬────────────────┐ │
-│ │ len <uvarint> │ encoding <1 byte> │ data │ CRC32 <4 byte> │ │
-│ └───────────────┴───────────────────┴──────┴────────────────┘ │
-└───────────────────────────────────────────────────────────────┘
+┌────────────────────────────┬─────────────────────┐
+│ magic(0x85BD40DD) <4 byte> │ version(1) <1 byte> │
+├────────────────────────────┴─────────────────────┤
+│ ┌──────────────────────────────────────────────┐ │
+│ │                   Chunk 1                    │ │
+│ ├──────────────────────────────────────────────┤ │
+│ │                     ...                      │ │
+│ ├──────────────────────────────────────────────┤ │
+│ │                   Chunk N                    │ │
+│ └──────────────────────────────────────────────┘ │
+└──────────────────────────────────────────────────┘
+```
+
+
+# Chunk
+
+```
+┌───────────────┬───────────────────┬──────────────┬────────────────┐
+│ len <uvarint> │ encoding <1 byte> │ data <bytes> │ CRC32 <4 byte> │
+└───────────────┴───────────────────┴──────────────┴────────────────┘
 ```