Clarify meaning of reference to ditabase docs

[robander]

The DITA community has been confused in the past about what it means to use a topic reference to a composite ditabase document (that is, one that has the root element <dita> with topics inside). Links to the document with no topic ID are equivalent to links to the first topic. There has been confusion about what it means for publishing (should a reference to the doc signify an intent to publish the whole doc, or the first topic). The original expectation was that the whole document should be published, but the linking language has caused confusion. Need to clarify this in DITA 2.0, as discussed April 3 and April 10 2018

[keberlein]

Discussed again at the DITA TC meeting on 08 May 2018. (Link to minutes: https://lists.oasis-open.org/archives/dita/201805/msg00014.html)

Major points for spec changes:

• Example in topic covering @href needs refinement
• Need clarity on how ditabase is processed when a fragment identifier is NOT present)