More precise wording about add/remove

Author: MichaelChirico

R/data.table.R

@@ -1195,7 +1195,7 @@ replace_dot_alias = function(e) {
           #   ok=-1 which will trigger setalloccol with verbose in the next
           #   branch, which again calls _selfrefok and returns the message then
           if ((ok<-selfrefok(x, verbose=FALSE))==0L)   # ok==0 so no warning when loaded from disk (-1) [-1 considered TRUE by R]
-            if (is.data.table(x)) warningf("A shallow copy of this data.table was taken so that := can add %d new columns by reference. At an earlier point, this data.table was copied by R (or was created manually using structure() or similar). Avoid names<- and attr<- which in R currently (and oddly) may copy the whole data.table. Use set* syntax instead to avoid copying: ?set, ?setnames and ?setattr. It's also common for data.table-unaware packages to produce affected tables. If this message doesn't help, please report your use case to the data.table issue tracker so the root cause can be fixed or this message improved.", length(newnames))
+            if (is.data.table(x)) warningf("A shallow copy of this data.table was taken so that := can add or remove %d columns by reference. At an earlier point, this data.table was copied by R (or was created manually using structure() or similar). Avoid names<- and attr<- which in R currently (and oddly) may copy the whole data.table. Use set* syntax instead to avoid copying: ?set, ?setnames and ?setattr. It's also not unusual for data.table-agnostic packages to produce tables affected by this issue. If this message doesn't help, please report your use case to the data.table issue tracker so the root cause can be fixed or this message improved.", length(newnames))
             # !is.data.table for DF |> DT(,:=) tests 2212.16-19 (#5113) where a shallow copy is routine for data.frame
           if ((ok<1L) || (truelength(x) < ncol(x)+length(newnames))) {
             DT = x  # in case getOption contains "ncol(DT)" as it used to.  TODO: warn and then remove