[MLIR][Doc] Prepend "Variadic of" in front of variadic operands (#69285)

Table of Operands for operations like:

https://mlir.llvm.org/docs/Dialects/MemRef/#operands-6

Don't distinguish variadic ODS operands from others right now.

After this change, it'll print:

| Operand      | Description       |
| dynamicSizes | Variadic of index |

instead of:

| Operand      | Description |
| dynamicSizes | index       |