[Rd] R-intro: function 'stderr' and 'sd'

[Martin Maechler]

>>>>> Suharto Anggono Suharto Anggono via R-devel <r-devel at r-project.org>
>>>>>     on Fri, 9 Sep 2016 16:52:01 +0000 writes:

    > In "An Introduction to R" Version 3.3.1, in "4.2 The function tapply() and ragged arrays", after
    > stderr <- function(x) sqrt(var(x)/length(x))  ,
    > there is a note in brackets:
    > Writing functions will be considered later in [Writing your own functions], and in this case was unnecessary as R also has a builtin function sd().

    > The part "in this case was unnecessary as R also has a builtin function sd()" is misleading. The builtin function sd() doesn't calculate standard error of the mean. It calculates standard deviation. The function 'stderr' can use 'sd':
    > function(x) sd(x)/sqrt(length(x))

You are right; thank you Suharto.
It now says

(Writing functions will be considered later in @ref{Writing your own
functions}.  Note that @R{}'s a builtin function @code{sd()} is something different.)