doc: add String manipulation functions

docs.md

@@ -4051,6 +4051,55 @@ metrics counters.
 - `events` : `integer`
 - `events_waiting` : `integer`
 
+## String manipulation functions
+
+These string utilities are needed internally for dealing with Windows, and are exported to allow clients to work uniformly with this data when the libuv API is not complete.
+
+**Notes**:
+
+1. New in luv version 1.49.0.
+2. A UTF-16 character is 2 bytes, and a UTF-8 character is 1 byte.
+3. Luv use Lua style string, which means that all inputs and return values (UTF-8 or UTF-16 strings) not include NUL terminated.
+
+### `uv.utf16_length_as_wtf8()`
+
+Get the length of a UTF-16 (or UCS-2) string `utf16` value after converting it to WTF-8.
+
+**Parameters:**
+- `utf16`: `string`
+- `len`: `integer` or `nil` (default: `#utf16/2`)
+
+**Returns:** `integer`
+
+### `uv.utf16_to_wtf8()`
+
+Convert UTF-16 (or UCS-2) string `utf16` to UTF-8 string. The `len` count (in characters)
+gives the length of utf16.
+
+**Parameters:**
+- `utf16`: `string`
+- `len`: `integer` or `nil` (default: `#utf16/2`)
+
+**Returns:** `string`
+
+### `uv.wtf8_length_as_utf16()`
+
+Get the length in characters of a WTF-8 `wtf8` value after converting it to UTF-16 (or UCS-2).
+
+**Parameters:**
+- `wtf8`: `string`
+
+**Returns:** `integer`
+
+### `uv.wtf8_to_utf16()`
+
+Convert WTF-8 string in `wtf8` to UTF-16 (or UCS-2) string.
+
+**Parameters:**
+- `wtf8`: `string`
+
+**Returns:** `string`
+
 ---
 
 [luv]: https://github.com/luvit/luv