rk.power: begin including documentation

git-svn-id: https://svn.code.sf.net/p/rkward/code@4887 7936002b-cd1d-0410-9ce3-d7ebf93b5162
rk.power: hint to df numerator/denominator

git-svn-id: https://svn.code.sf.net/p/rkward/code@4888 7936002b-cd1d-0410-9ce3-d7ebf93b5162
Explicit markup of NAs in data editor.

git-svn-id: https://svn.code.sf.net/p/rkward/code@4889 7936002b-cd1d-0410-9ce3-d7ebf93b5162