nobs.default( cov.wt(.) )  now also works, 
and so also for objects "containing" a cov.wt()


git-svn-id: https://svn.r-project.org/R/trunk@66992 00db46b3-68df-0310-9c12-caf00c1e9a41
Explain special-casing of main effects.  (See reverted r66983 for a more aggressive response to PR#16070.) 

git-svn-id: https://svn.r-project.org/R/trunk@66993 00db46b3-68df-0310-9c12-caf00c1e9a41