Update docs

docs/en/sql-reference/functions/string-functions.md

@@ -577,26 +577,52 @@ Like `concatWithSeparator` but assumes that `concatWithSeparator(sep, expr1, exp
 
 A function is called injective if it returns for different arguments different results. In other words: different arguments never produce identical result.
 
-## substring(s, offset, length)
+## substring
 
-Returns a substring with `length` many bytes, starting at the byte at index `offset`. Character indexing starts from 1. Can be also used with [Enum](../../sql-reference/data-types/enum.md) types.
+Returns the substring of a string `s` which starts at the specified byte index `offset`. Byte counting starts from 1. If `offset` is 0, an empty string is returned. If `offset` is negative, the substring starts `pos` characters from the end of the string, rather than from the beginning. An optional argument `length` specifies the maximum number of bytes the returned substring may have.
 
 **Syntax**
 
 ```sql
-substring(s, offset, length)
+substring(s, offset[, length])
 ```
 
 Alias:
 - `substr`
 - `mid`
 
+**Arguments**
+
+- `s` — The string to calculate a substring from. [String](../../sql-reference/data-types/string.md), [FixedString](../../sql-reference/data-types/fixedstring.md) or [Enum](../../sql-reference/data-types/enum.md)
+- `offset` — The starting position of the substring in `s` . [(U)Int*](../../sql-reference/data-types/int-uint.md).
+- `length` — The maximum length of the substring. [(U)Int*](../../sql-reference/data-types/int-uint.md). Optional.
+
+**Returned value**
+
+A substring of `s` with `length` many bytes, starting at index `offset`.
+
+Type: `String`.
+
+**Example**
+
+``` sql
+SELECT 'database' AS db, substr(db, 5), substr(db, 5, 1)
+```
+
+Result:
+
+```result
+┌─db───────┬─substring('database', 5)─┬─substring('database', 5, 1)─┐
+│ database │ base                     │ b                           │
+└──────────┴──────────────────────────┴─────────────────────────────┘
+```
+
 ## substringUTF8
 
 Like `substring` but for Unicode code points. Assumes that the string contains valid UTF-8 encoded text. If this assumption is violated, no exception is thrown and the result is undefined.
 
 
-## substringIndex(s, delim, count)
+## substringIndex
 
 Returns the substring of `s` before `count` occurrences of the delimiter `delim`, as in Spark or MySQL.
 
@@ -627,7 +653,7 @@ Result:
 └──────────────────────────────────────────────┘
 ```
 
-## substringIndexUTF8(s, delim, count)
+## substringIndexUTF8
 
 Like `substringIndex` but for Unicode code points. Assumes that the string contains valid UTF-8 encoded text. If this assumption is violated, no exception is thrown and the result is undefined.