clean up refpurpose contents to be consistent

This is to be consistent with other manuals:
 - no period punctuation at end
 - no capitalization at beginning unless acronym or proper noun
 - and no simple article (like "A") to start sentence.

While the docbook specification says refpurpose is a "one sentence",
its examples show it with the rules shown above.

Also compared with many manpages as this is common format for the
.Nd macro's one line description.

bin/python/dnssec-checkds.docbook

@@ -32,7 +32,7 @@
 
   <refnamediv>
     <refname><application>dnssec-checkds</application></refname>
-    <refpurpose>A DNSSEC delegation consistency checking tool.</refpurpose>
+    <refpurpose>DNSSEC delegation consistency checking tool</refpurpose>
   </refnamediv>
 
   <docinfo>