Clean up documentation of common elements

When DIB_COMMON_ELEMENTS was added, some of the documentation no
longer made sense where it was.  This moves the documentation of
the common elements to the spot where DIB_COMMON_ELEMENTS is
defined.

Change-Id: Icd4a3eddcf95726989527f56075dde99f2ae5ca7
Merge "Clean up documentation of common elements"