`nimSwitch` needs better documentation

[perrydv]

The man page says to look in the manual, and the manual only documents the translation switch -> nimSwitch, so it is effectively undocumented.

[paciorek]

@perrydv it's not clear to me nimSwitch is user-facing. It seems to require integer-valued arguments, so less flexible than switch, and the first argument must be code and not a literal (as I just found) out.

nf <-nimbleFunction(
    run = function(myval = integer()) {
        returnType(double())
        nimSwitch(2L, 1:3,  # failure if first arg is not a variable
                  out <- 3,
                  out<-4,
                  out<-5)
        return(out)
    })
cnf <- compileNimble(nf)

Compiling
  [Note] This may take a minute.
  [Note] Use 'showCompilerOutput = TRUE' to see C++ compilation details.
Error in code$args[[1]]$name : $ operator is invalid for atomic vectors

[paciorek]

My current thought is that we take mention of it out of the manual and (if this passes R CMD check) do not have ?nimSwitch redirect to our generic "documentation" for math functions. I would also be inclined not to export nimSwitch but don't want to change that for fear of unintended consequences.