Documented `replace` function.

I left the possibility of passing unquoted parameters undocumented since there's a few things 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
```