Declare custom javadoc tags

Declaring javadoc tags in the configuration for maven-javadoc-plugin
resolves several build warnings that appeared when using the
sonatype-oss-release profile.

Change-Id: I1be6950dd43fe844107a7f23381f5d7b20fe2935
Update to docbkx maven plugin 2.0.15

The primary advantage of the new release is it supports specifying the
encoding for generated files, which was actually causing problems in
previous releases. The encoding is implicitly specified by setting the
project.build.sourceEncoding property (which is already in place).

Change-Id: I7cd778c6c98e23b4d92d114ef167adcfd30213b9
Merge "Declare custom javadoc tags"
Merge "Update to docbkx maven plugin 2.0.15"