tsdb/docs: fix head chunks directory + link from README (#7309)

Signed-off-by: Simon Pasquier <spasquie@redhat.com>

tsdb/docs/format/README.md

@@ -2,5 +2,6 @@
 
 * [Index](index.md)
 * [Chunks](chunks.md)
+* [Head Chunks](head_chunks.md)
 * [Tombstones](tombstones.md)
 * [Wal](wal.md)

tsdb/docs/format/head_chunks.md

@@ -1,7 +1,7 @@
 # Head Chunks on Disk Format
 
-The following describes the format of a chunks file,
-which is created in the `wal/chunks/` inside the data directory.
+The following describes the format of a chunks file, which is created in the
+`chunks_head/` directory inside the data directory.
 
 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).
@@ -27,10 +27,13 @@ in-file offset (lower 4 bytes) and segment sequence number (upper 4 bytes).
 
 # Chunk
 
-Unlike chunks in the on-disk blocks, here we additionally store series reference that the chunks belongs to and the mint/maxt of the chunks. This is because we don't have an index associated with these chunks, hence these meta information are used while replaying the chunks.
+Unlike chunks in the on-disk blocks, here we additionally store series
+reference that the chunks belongs to and the mint/maxt of the chunks. This is
+because we don't have an index associated with these chunks, hence these meta
+information are used while replaying the chunks.
 
 ```
 ┌─────────────────────┬───────────────────────┬───────────────────────┬───────────────────┬───────────────┬──────────────┬────────────────┐
 | series ref <8 byte> | mint <8 byte, uint64> | maxt <8 byte, uint64> | encoding <1 byte> | len <uvarint> | data <bytes> │ CRC32 <4 byte> │
 └─────────────────────┴───────────────────────┴───────────────────────┴───────────────────┴───────────────┴──────────────┴────────────────┘
-```
+```