Currently the only value that can be used in Content Specs is "DocBook 4.5", however once all other components have been implemented for BZ#1051919, we'll need to add "DocBook 5" as a valid option.
Also with this we need to ensure DocBook 4.5 and DocBook 5.0 topics aren't mixed in a Content Spec as they aren't compatible (mainly due to DocBook 4.5 using DTD's and DocBook 5.0 using a schema)
Added in 1.4-SNAPSHOT build 201402071538 DocBook 5.0 can now be used as a DTD/Format option. I've also changed the "DTD" metadata name to "Format", to better describe the the metadata (since DocBook 5.0 uses Relax NG). This also allows for future additions (ie possibly AsciiDoc). Note: This can be tested on the test/development server.
Confirmed that the Format metadata element can be set to DocBook 5.0, and that topics with xml formats that do not match are identified as validation errors.