add documentation

docs/syntax/packet.md

@@ -20,6 +20,7 @@ This diagram type is particularly useful for developers, network engineers, educ
 packet-beta
 start: "Block name" %% Single-bit block
 start-end: "Block name" %% Multi-bit blocks
+16bits: "Block name" %% Auto appends to previous block without specifying start
 ... More Fields ...
 ```
 
@@ -82,6 +83,16 @@ title UDP Packet
 48-63: "Checksum"
 64-95: "Data (variable length)"
 ```
+Or you can write this as
+```mermaid-example
+packet-beta
+title UDP Packet
+16bits: "Source Port"
+16bits: "Destination Port"
+16bits: "Length"
+16bits: "Checksum"
+64-95: "Data (variable length)"
+```
 
 ```mermaid
 packet-beta