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

doc/src/sgml/config.sgml

@@ -2724,6 +2724,8 @@ include_dir 'conf.d'
        <listitem>
         <para>
          Controls the largest I/O size in operations that combine I/O.
+         If this value is specified without units, it is taken as blocks,
+         that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
          The default is 128kB.
         </para>
        </listitem>