some documentation fixes (#640)

* a few more files to ignore

* the pgtune we reference seems to be dead, provide
a few alternatives.

* steal pod syntax check from snmp::info, but don't bother with checking Test::Pod version.
we want 1.22 or later, which was released in 2005

* reference the release notes in our wiki. changes you need to make manually during upgrades are documented there.

* add pod test to manifest

* term was defined around line 111 and then again at line 209, rename the second one

xt/01-local-pod.t

@@ -0,0 +1,8 @@
+#!/usr/bin/env perl
+
+use strict;
+use warnings;
+use Test::More;
+use Test::Pod;
+
+all_pod_files_ok();