Figured it out. MASS has a custom INDEX file ...
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1.1.4 The INDEX file
The optional file INDEX contains a line for each sufficiently
interesting object in the package, giving its name and a description
(functions such as print methods not usually called explicitly might not
be included). Normally this file is missing and the corresponding
information is automatically generated from the documentation sources
(using tools::Rdindex()) when installing from source.
The file is part of the information given by library(help = pkgname).
Rather than editing this file, it is preferable to put customized
information about the package into an overview help page (see
Documenting packages) and/or a vignette (see Writing package vignettes).
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On 2018-05-18 03:44 PM, Ben Bolker wrote:
>
> I notice that when I say help(package="MASS") I get separate indices
> for functions and data sets. AFAICT this doesn't seem to occur in other
> packages that have both functions and data sets (e.g. mgcv, lattice,
> lme4), despite the tags \docType{data} and \keywords{datasets} being
> used in the relevant .Rd files; I don't see any other obvious magic in
> the .Rd files for MASS (in fact, they don't use the \docType{} tag at
> all ...
>
> Before I go spelunking, does anyone have any guesses/ideas/information
> about why MASS is special (or if it is)?
>
> cheers
> Ben Bolker
>
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