document substr(bytea)

docs/sql/functions-operators/sql-function-binarystring.md

@@ -143,3 +143,27 @@ SELECT sha512('risingwave'::bytea);
 \x3d6d6078c75ad459cdc689216d5de35bd6d9a9a50b9bed96417aaf7ad25057b37460564f0ad23a589c655eda45026096a6bab08b3c863f0425cbfea64b5f84a8
 ```
 
+---
+
+### `substr`
+
+Extracts a substring from a binary string (bytea) starting at position `start_int` for `count_int` bytes. If `count_int` is omitted, the substring extends to the end of the bytea value.
+
+```sql title=Syntax
+substr ( bytea_value, start_int, [, count_int] ) -> bytea
+```
+
+```sql title=Example
+SELECT substr('abcde'::bytea, 2, 7);
+```
+```
+\x62636465
+```
+
+```sql title=Example
+SELECT substr('abcde'::bytea, -2, 5);
+```
+```
+\x6162
+```
+---  

docs/sql/functions-operators/sql-function-conditional.md

@@ -111,4 +111,3 @@ Returns null if *value1* equals to *value2*, otherwise returns *value1*.
 ```sql
 NULLIF ( value1, value2 )
 ```
-

docs/sql/functions-operators/sql-function-string.md

@@ -519,7 +519,7 @@ starts_with('RisingWave is powerful', 'Rising') → true
 
 ### `substr`/`substring`
 
-Extracts the substring from input_string starting at position start_int and extending for count_int characters, if specified. start_int should be equal to or larger than 1.
+Extracts the substring from input_string starting at position start_int and extending for count_int characters.
 
 ```bash title=Syntax
 substr( input_string, start_int[, count_int] ) → output_string