Add --buffer-usage-limit option to vacuumdb

1cbbee033 added BUFFER_USAGE_LIMIT to the VACUUM and ANALYZE commands, so
here we permit that option to be specified in vacuumdb.

In passing, adjust the documents for vacuum_buffer_usage_limit and the
BUFFER_USAGE_LIMIT VACUUM option to mention "kB" rather than "KB".  Do the
same for the ERROR message in ExecVacuum() and
check_vacuum_buffer_usage_limit().  Without that we might tell a user that
the valid minimum value is 128 KB only to reject that because we accept
only "kB" and not "KB".

Also, add a small reminder comment in vacuum.h to try to trigger the
memory of anyone adding new fields to VacuumParams that they might want to
consider if vacuumdb needs to grow a new option too.

Author: Melanie Plageman
Reviewed-by: Justin Pryzby
Reviewed-by: David Rowley
Discussion: https://postgr.es/m/ZAzTg3iEnubscvbf@telsasoft.com

doc/src/sgml/ref/vacuumdb.sgml

@@ -278,6 +278,19 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>--buffer-usage-limit <replaceable class="parameter">buffer_usage_limit</replaceable></option></term>
+      <listitem>
+       <para>
+        Specifies the
+        <glossterm linkend="glossary-buffer-access-strategy">Buffer Access Strategy</glossterm>
+        ring buffer size for a given invocation of <application>vacuumdb</application>.
+        This size is used to calculate the number of shared buffers which will
+        be reused as part of this strategy.  See <xref linkend="sql-vacuum"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>-n <replaceable class="parameter">schema</replaceable></option></term>
       <term><option>--schema=<replaceable class="parameter">schema</replaceable></option></term>