Issue 12858:  Document opEquals usage in AAs

AA runtime has changed to use `opEquals` instead of `opCmp`. Also the compiler has been adjusted to reject AA struct keys that only define `opCmp` and not `opEquals` to avoid regressions. These documentation changes reflect those changes.

Note, I did NOT build the docs (I don't know how!). Please take care when reviewing to ensure I did all the macros correctly.