[Issue 18998] Improve Operator Overloading docs

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18998

--- Comment #4 from Nick Treleaven <nick at geany.org> ---
This one adds examples for opCmp, opOpAssign, s.opIndex(s.opSlice(...)).
https://github.com/dlang/dlang.org/pull/3467

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