[MLIR][NVVM] [NFC] Update Docs for shfl.sync Op (#89044)

The first argument to the nvvm_shfl_sync_* family
of intrinsics is the thread_mask (aka member_mask). 
This patch renames the corresponding operand in the Op
to reflect the same i.e. `dst` -> `thread_mask`.

While we are there, add summary and description
for this Op.

Signed-off-by: Durgadoss R <durgadossr@nvidia.com>