[flang] Update the description of `!fir.coordinate_of`

This change was suggested in one of the comments for
https://reviews.llvm.org/D115333. Basically, the following usage is
valid, but the current wording suggests otherwise:
```
%1 = fir.coordinate_of %a, %k : (!fir.ref<!fir.array<10 x 10 x i32>>, index) -> !fir.ref<!fir.array<10 x i32>>
```
A test is also added to better document this particular case.

Differential revision: https://reviews.llvm.org/D115929