RFR: JDK-8238506 fix obsolete comments and inconsistent exceptions in BaseTaglet

[Jonathan Gibbons]

Please review a small cleanup of internal doc comments, and nearby code, 
triggered by other recent review comments.

The focus of the change was the obsolete references  to 
'<code>Tag</code>' where 'Tag' was the JDK8 predecessor to using DocTree 
in javadoc.  The comments are updated. The other changes in these files 
were IDE cleanup suggestions.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8238506
Webrev: http://cr.openjdk.java.net/~jjg/8238506/webrev/