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)
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 rel
