Add some string functions document (#928)

docs/documentation/cn/sql-reference/sql-functions/string-functions/lower.md

@@ -0,0 +1,20 @@
+# lower
+
+## Syntax
+
+`INT lower(VARCHAR str)`
+
+## Description
+
+将参数中所有的字符串都转换成小写
+
+## Examples
+
+```
+mysql> SELECT lower("AbC123");
++-----------------+
+| lower('AbC123') |
++-----------------+
+| abc123          |
++-----------------+
+```