tweaks

git-svn-id: https://svn.r-project.org/R/trunk@62709 00db46b3-68df-0310-9c12-caf00c1e9a41
tweak

git-svn-id: https://svn.r-project.org/R/trunk@62710 00db46b3-68df-0310-9c12-caf00c1e9a41
more tweaks to serialization, document

git-svn-id: https://svn.r-project.org/R/trunk@62712 00db46b3-68df-0310-9c12-caf00c1e9a41