Improve comment/doc for pg_dump_options config

Author: Julian Vanden Broeck

pg_back.conf

@@ -106,7 +106,10 @@ purge_min_keep = 0
 # Number of pg_dump commands to run concurrently.
 jobs = 1
 
-# inject these options to pg_dump
+# This parameter specifies additional command-line options that will be passed
+# directly to the pg_dump commands. Some essential options (such as format and
+# connection), are automatically included and should not be specified. pg_back
+# verbose mode can help identify which options are used.
 pg_dump_options =
 
 # When dumping from a hot standby server, wait for exclusive locks to