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

@@ -2697,6 +2697,8 @@ include_dir 'conf.d'
          This parameter can only be set in
          the <filename>postgresql.conf</filename> file or on the server
          command line.
+         If this value is specified without units, it is taken as blocks,
+         that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
          The maximum possible size depends on the operating system and block
          size, but is typically 1MB on Unix and 128kB on Windows.
          The default is 128kB.
@@ -2716,6 +2718,8 @@ include_dir 'conf.d'
          higher than the <varname>io_max_combine_limit</varname> parameter, the
          lower value will silently be used instead, so both may need to be raised
          to increase the I/O size.
+         If this value is specified without units, it is taken as blocks,
+         that is <symbol>BLCKSZ</symbol> bytes, typically 8kB.
          The maximum possible size depends on the operating system and block
          size, but is typically 1MB on Unix and 128kB on Windows.
          The default is 128kB.