[Request] Add note to ?setorder that character ordering sorts case, then alphabetically

[HughParsonage]

I noticed that setorder is case-sensitive in a different way to base::order

DT <- data.table(x = c("a", "A", "B", "b"), y = 1:4)
DF <- as.data.frame(DT)

# ordered by case, then alphabetically and capital letters occur before lower-case letters
DT[order(x)]
#    x y
# 1: A 2
# 2: B 3
# 3: a 1
# 4: b 4

# in alphabetical order first, then case-sensitively (and lower-case first)
DF[order(DF$x), ]
#   x y
# 3 a 1
# 1 A 2
# 4 b 4
# 2 B 3

If this is intended behaviour, I believe it should be added to the documentation for ?setorder.

[MichaelChirico]

I believe this is documented, though perhaps it could be done so more elegantly/obviously:

data.table always reorders in C-locale. To sort by session locale, use x[base::order(.)] instead.

Also, potential duplicate of #565? But definite duplicate of #1103.

[HughParsonage]

FWIW, I saw data.table always reorders in C-locale in the documentation (before raising this issue) but didn't realize it had that effect. (I thought it was just a comment about performance.)