update re data.table changes

[franknarf1]

Here are some:

• CJ no longer requires explicit names CJ now auto-names its input Rdatatable/data.table#2977
• In constructing or debugging x[i] calls, recommend x[, print(i)], with print being necessary in some cases due to new-ish convenience features of j (example: reconsider treating DT[!int] as DT[-int]? Rdatatable/data.table#3699 (comment)) hm, more simple would be just with(DT, z) -- then there is no need to deal with j being transformed to a list of columns / data.table either
• alloc.col (noted by name in the tutorial) to be called setalloccol, soon. I should also check the latest faqs or vignettes to see if guidance has changed