doc: clarify that max_wal_size is "during" checkpoints

Previous wording was "between".

Reported-by: Pavel Luzanov

Discussion: https://postgr.es/m/26906a54-d7cb-2f8e-eed7-e31660024694@postgrespro.ru

Backpatch-through: 9.5

doc/src/sgml/config.sgml

@@ -3219,7 +3219,7 @@ include_dir 'conf.d'
       </term>
       <listitem>
        <para>
-        Maximum size to let the WAL grow to between automatic WAL
+        Maximum size to let the WAL grow during automatic
         checkpoints. This is a soft limit; WAL size can exceed
         <varname>max_wal_size</varname> under special circumstances, such as
         heavy load, a failing <varname>archive_command</varname>, or a high