[MLIR] [docs] Fix misguided examples in memref.subview operation.

The examples in `memref.subview` operation are misguided in that subview's strides operands mean "memref-rank number of strides that compose multiplicatively with the base memref strides in each dimension.".
So the below examples should be changed from `Strides: [64, 4, 1]` to `Strides: [1, 1, 1]`

Before changes
```
// Subview with constant offsets, sizes and strides.
%1 = memref.subview %0[0, 2, 0][4, 4, 4][64, 4, 1]
  : memref<8x16x4xf32, (d0, d1, d2) -> (d0 * 64 + d1 * 4 + d2)> to
    memref<4x4x4xf32, (d0, d1, d2) -> (d0 * 64 + d1 * 4 + d2 + 8)>
```

After changes
```
// Subview with constant offsets, sizes and strides.
%1 = memref.subview %0[0, 2, 0][4, 4, 4][1, 1, 1]
  : memref<8x16x4xf32, (d0, d1, d2) -> (d0 * 64 + d1 * 4 + d2)> to
    memref<4x4x4xf32, (d0, d1, d2) -> (d0 * 64 + d1 * 4 + d2 + 8)>
```

Also I fixed some syntax issues in docs related with memref layout map and added detailed explanation in subview rank reducing case.

Reviewed By: herhut

Differential Revision: https://reviews.llvm.org/D114500