Closes # 1699, subtle point of explication in melt dox

whoops

NEWS.md

@@ -82,6 +82,8 @@
 
 5. Printing with `quote = TRUE` now quotes column names as well, [#1319](https://github.com/Rdatatable/data.table/issues/1319). Thanks @jan-glx for the suggestion and @MichaelChirico for the PR.
 
+6. Added a blurb to `?melt.data.table` explicating the subtle difference in behavior of the `id.vars` argument vis-a-vis its analog in `reshape2::melt`, [#1699](https://github.com/Rdatatable/data.table/issues/1699). Thanks @MichaelChirico for uncovering and filing. 
+
 
 ### Changes in v1.10.4  (on CRAN 01 Feb 2017)
 

man/melt.data.table.Rd

@@ -22,7 +22,7 @@ load \code{reshape2} package \emph{before} loading \code{data.table}.
 \item{data}{ A \code{data.table} object to melt.}
 \item{id.vars}{vector of id variables. Can be integer (corresponding id 
 column numbers) or character (id column names) vector. If missing, all 
-non-measure columns will be assigned to it.}
+non-measure columns will be assigned to it. If integer, must be positive; see Details. }
 \item{measure.vars}{vector of measure variables. Can be integer (corresponding 
 measure column numbers) or character (measure column names) vector. If missing, 
 all non-id columns will be assigned to it.
@@ -77,7 +77,10 @@ be coerced to \code{character} type. This is to be compatible with
 \code{reshape2}'s \code{melt.data.frame}. To get a \code{factor} column, set 
 \code{value.factor = TRUE}. \code{melt.data.table} also preserves 
 \code{ordered} factors.
+
+Note that, as opposed to the (undocumented) behavior of \code{reshape2::melt}, \code{id.vars}, when specified as numbers, must be between 1 and \code{ncol(data)}.
 }
+
 \value{
 An unkeyed \code{data.table} containing the molten data.
 }