[Rd] Roxygen

[Peter Danenberg]

> It is desirable that these tags and their syntax and semantics be
> compatible with those used by Doxygen and Javadoc, but extended with
> tags important for R specific things.

Doxygen, in particular, has some tags that are meaningless in R
(@category and @protocol come to mind); wouldn't it be worthwhile,
therefore, to start with a subset of Rd?

Thinks like @name and @format could be gleaned from the code; and it
seems like preserving other Rd names would uphold the "principle of
least surprise."