doc/radosgw/s3select: add SQL LIMIT operator examples

doc/radosgw/s3select: add SQL LIMIT operator examples

This commit adds concrete examples for the SQL LIMIT operator section,
demonstrating both non-aggregation queries (retrieving first N rows)
and aggregation queries with row processing limits.

Signed-off-by: Khushhal <Khushhalsinha0909@gmail.com>
doc/radosgw/s3select: fix RST formatting in LIMIT examples

- Add empty line before example section for proper paragraph separation
- Wrap lines at 80 characters maximum for consistency
- Add empty line after final paragraph

Signed-off-by: Khushhal <Khushhalsinha0909@gmail.com>