[Rd] NA in doc for options(matprod="default")

[Serguei Sokol]

Hi,

A colleague of mine has spotted me a passage of the doc ?option talking 
about Inf and NaN check in 'matprod=default' section:
https://stat.ethz.ch/R-manual/R-devel/library/base/html/options.html

I am wondering if NA should be mentioned too as the check seems to 
include this "value" too. NA being different from Inf and NaN it is 
worth mentioning, isn't it?

Best,
Serguei.