I don't know how well RFC 1855 reflected common practice, though. It might be worthwhile to check the rendering code in clients like tin and mutt.
According to my own local Markdown formatters, the answer would be both "usr" and "bin", with the surrounding slashes removed, but the internal slash remaining. In other words:
usr/bin
(but underlined instead of italic!)
Of course, this problem is nothing new since a filename might easily be named `_my_file_`.