An rd_section represents an Rd command that can appear at the top-level
of an Rd document, like \name{}, \title{}, \description{}, or
\section{}.
Methods
If provide your own rd_section type, you'll also need to define a
format.rd_section_{type} method that returns formatted Rd output. You
may also need to provide a merge.rd_section_{type} method if two
sections can not be combined with rd_section(x$type, c(x$value, y$value)).
See vignette("extending") for more details.
