Escaping `[`, `]` character outside of code blocks is buggy in markdown mode

[mb706]

Trying to escape [ characters outside of code blocks generates broken .Rd files.

Examples (in the description block, but the bug occurs in other blocks as well):

#' \[ test \] description

generates

\description{
[ test ] description

[ test ]: R:%20test%20%5C
}

A way around this is currently to only escape the beginning [ character:

#' \[ test ] description

to get

\description{
[ test ] description
}

However, I have not found a way to put a link between escaped brackets (without resorting to \link):

#' \[ [test] ] description

gives (also when escaping both outer [ and ]):

\description{
[ [test] ] description

[ [test]: R:%20[test
}

[gaborcsardi]

Reproducible example:

roc <- rd_roclet()
out1 <- roc_proc_text(roc, "
  #' Title
  #'
  #' \\[ test \\] description
  #' @md
  foo <- function() {}")[[1]]
out1

#> % Generated by roxygen2: do not edit by hand
#> % Please edit documentation in RtmpZ0IFGm/file125e27337212c
#> \name{foo}
#> \alias{foo}
#> \title{Title}
#> \usage{
#> foo()
#> }
#> \description{
#> [ test ] description [ test ]: R:%20test%20%5C
#> }