doc: mention archive_command and recovery_command are exec'ed locally

Report by Craig Ringer

doc/src/sgml/config.sgml

@@ -2279,7 +2279,7 @@ include 'filename'
       </indexterm>
       <listitem>
        <para>
-        The shell command to execute to archive a completed WAL file
+        The local shell command to execute to archive a completed WAL file
         segment.  Any <literal>%p</> in the string is
         replaced by the path name of the file to archive, and any
         <literal>%f</> is replaced by only the file name.

doc/src/sgml/recovery-config.sgml

@@ -42,7 +42,7 @@
       </indexterm>
       <listitem>
        <para>
-        The shell command to execute to retrieve an archived segment of
+        The local shell command to execute to retrieve an archived segment of
         the WAL file series. This parameter is required for archive recovery,
         but optional for streaming replication.
         Any <literal>%f</> in the string is