[Rd] pch=NA in bxp.Rd (PR#8073)

[arnima@u.washington.edu]

I'd like to iterate my earlier request (#7737) to change the documentation 
for bxp().

The argument outpch=" " needs to be replaced with outpch=NA in two places.

I actually wrote this part of the documentation myself at one point, but 
have now realized that pch=NA and pch=" " are not the same:

   x <- split(rlnorm(26e4), letters)
   ## NA generates small file
   postscript("na.ps");    boxplot(x,outpch=NA);  dev.off()
   ## Space generates large file
   postscript("space.ps"); boxplot(x,outpch=" "); dev.off()

Thank you,
Arni

R 2.1.1-patched on WinXP