doc: better explain where pg_bindir won't be applied

Basically any setting which can contain a user-defined script
*must* have the full path set, even if it's repmgr being executed.

We could potentially apply some heuristics to detect if the first
item in the setting is "repmgr" (or more precisely repmgrd's program
name), but this will require some careful thought and testing
that it works as intended.

doc/configuration-file-service-commands.sgml

@@ -48,6 +48,13 @@
     service_reload_command</programlisting>
   </para>
 
+  <note>
+    <para>
+      &repmgr; will not apply <option>pg_bindir</option> when executing any of these commands;
+      these can be user-defined scripts so must always be specified with the full path.
+    </para>
+  </note>
+
   <note>
     <para>
       It's also possible to specify a <varname>service_promote_command</varname>.