Fix function comment for dumpACL()

The comment for dumpACL() got neglected when initacls and initracls were
added and the discussion of what 'racls' is wasn't very clear either.

Per complaint from Tom.

src/bin/pg_dump/pg_dump.c

@@ -14435,10 +14435,20 @@ dumpDefaultACL(Archive *fout, DefaultACLInfo *daclinfo)
  * 'tag' is the tag for the archive entry (typ. unquoted name of object).
  * 'nspname' is the namespace the object is in (NULL if none).
  * 'owner' is the owner, NULL if there is no owner (for languages).
- * 'acls' is the string read out of the fooacl system catalog field;
- *		it will be parsed here.
- * 'racls' contains any initial ACLs that the object had which have now been
- *		revoked by the user, it will also be parsed here.
+ * 'acls' contains the ACL string of the object from the appropriate system
+ * 		catalog field; it will be passed to buildACLCommands for building the
+ * 		appropriate GRANT commands.
+ * 'racls' contains the ACL string of any initial-but-now-revoked ACLs of the
+ * 		object; it will be passed to buildACLCommands for building the
+ * 		appropriate REVOKE commands.
+ * 'initacls' In binary-upgrade mode, ACL string of the object's initial
+ * 		privileges, to be recorded into pg_init_privs
+ * 'initracls' In binary-upgrade mode, ACL string of the object's
+ * 		revoked-from-default privileges, to be recorded into pg_init_privs
+ *
+ * NB: initacls/initracls are needed because extensions can set privileges on
+ * an object during the extension's script file and we record those into
+ * pg_init_privs as that object's initial privileges.
  *----------
  */
 static void