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A general description is here.
the refman.pdf works around a problem with the indexing with hyperref 6.79f.
pchisq(x, df, ncp=L) no longer can return values slightly larger
than one for large L.
pgamma(x, a, lower.tail=FALSE) and qgamma(..) are now considerably
qchisq(p, df, ncp, lower.tail = FALSE) with ncp >= 80 was inaccurate for small p (as the help page said): it is now less inaccurate. (In part, PR#13999.)
For ncp less than but close to 80, pchisq() and qchisq() are more accurate for probablilities very close to 1 (a series expansion was truncated slightly too early).
pchisq(x, df, ncp) no longer can return values just larger than one for large values of ncp.
The internal record-keeping by dyn.load/dyn.unload was incomplete, which
this could crash R if a DLL that registered .External routines had earlier been unloaded.