compile_dll() help-page is outdated

[assaron]

Currently, compiler_dll() help page says:

#' @param debug If `TRUE`, and if no user Makevars is found, then the build
#'   runs without optimisation (`-O0`) and with debug symbols (`-g`). See
#'   [compiler_flags()] for details. If you have a user Makevars (e.g.,
#'   `~/.R/Makevars`) then this argument is ignored.

It is no longer true since dd8ca4e Presence of Makevars does not influence anything anymore. As I understand, the behavior can now be controlled through some options and environmental variables.

[gaborcsardi]

True, would you like to submit a PR? (No pressure!)

[assaron]

@gaborcsardi I'm not 100% sure what's the intended behavior of this option and whether the current behavior matches that intention. In particular, I don't understand why the previous behavior was changed in the first place.