Improve javadoc on SortSpec to inform use of preferredName as a unique key

[nknize]

PR #1856 enhanced the SearchPlugin to enable pluggable SortBuilders. The mechanism uses a new SortSpec to define the specification of the builder and parser and exposes through the named registry. This named registry keys the sort specification off the preferredName which is required to be unique. Contributors may not be aware of other sort builders that share the same name.

This issue is to improve the javadocs to provide better documentation of this requirement.

relates #221

[andrross]

Resolved by #1909