DDSXTY12 — lookup_topicdescription semantics make it unusable

Section 7.6.3.2 specifies that lookup_topicdescription returns 1 matching representation. On multiple matches, an arbitrary representation is returned.

This is not helpful in any way for an application trying to access a certain representation. Either its signature should be changed to return a sequence of topic representations, or it should cyclically go over each representation and return 1 at a time, so that subsequent invocations may return different representations.

Clarified semantics of lookup_topicdescription

Added clarifications in section 7.6.3.2. and enhanced PIM-to-PSM mapping for this operation as described in the revised text.