add some references

git-svn-id: https://svn.r-project.org/R/trunk@67094 00db46b3-68df-0310-9c12-caf00c1e9a41
fixes for changes introduced in r67058 (S4 data.frame handling of [<- and [[<-)

git-svn-id: https://svn.r-project.org/R/trunk@67095 00db46b3-68df-0310-9c12-caf00c1e9a41
minor typo


git-svn-id: https://svn.r-project.org/R/trunk@67096 00db46b3-68df-0310-9c12-caf00c1e9a41