Documented `replace` function.

I left the possibilty of passing unquoted parameters undocumented since there're a few thing we may decide to change/fix later. E.g.:
```
1: replace(bar-1, 1, 2); // OK
2: replace(1, 1, 2); // Err. (first arg can't be a number)
3: replace(bar-1, 1, 2px); // OK but results in bar-2 not bar-2px
```