Automatic translation of opUnary!"++" into opOpAssign!"+"

[Francesco Cattoglio]

So, while I was studying the apropriate template constraints for 
my shiny new iota implementation, I found out this funny thing:

import std.stdio;
class Test{
    int x = 41;
    Test opOpAssign(string op)(int rhs) if (op == "+") {
         x += rhs;
         return this;
    }
}

void main() {
    Test t1 = new Test;
    //class Test has no opUnary defined, so the following
    //gets automagically converted into (t1) += (1)
    ++t1;
    writeln(t1.x); //prints 42, correct!
}

This actually comes really handy, but I couldn't find it into the 
language documentation on dlang.org, so it surprised me.
Did I miss it in the language specification? Should we add it 
somewhere to the docs?
Anyone with some spare time care to explain briefly what was the 
rationale behind this?