Add comment explaining lazy declared versions
Add comment explaining lazy declared versions

A recent change moved computing declared versions from using reflection
which occurred repeatedly to a lazily-initialized holder so that
declared versions are computed exactly once. This commit adds a comment
explaining the motivation for this change.

vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra