doc: Clarify units for io_combine_limit and io_max_combine_limit.

doc: Clarify units for io_combine_limit and io_max_combine_limit.

If these parameters are set without units, the values are interpreted
as blocks. This detail was previously missing from the documentation,
so this commit adds it.

Backpatch to v17 where io_combine_limit was added.

Author: Karina Litskevich <litskevichkarina@gmail.com>
Reviewed-by: Chao Li <lic@highgo.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CACiT8iZCDkz1bNYQNQyvGhXWJExSnJULRTYT894u4-Ti7Yh6jw@mail.gmail.com
Backpatch-through: 17