[Rd] base R pipe documentation

[Ben Bolker]

   As of right now, as far as I can tell, the documentation for the new 
native |> pipe still says that it's experimental.

https://github.com/wch/r-source/blob/trunk/src/library/base/man/pipeOp.Rd#L45

      *Pipe support is experimental and may change prior to release.*

Also still in the 4-1 branch:

https://github.com/wch/r-source/blob/R-4-1-branch/src/library/base/man/pipeOp.Rd#L45

   (The corresponding comment in the NEWS file has been fixed in the 
last 24 hours, but hasn't propagated to the online/HTML version on the 
developer page yet ...)

   As a "wish list" item, if there are any particularly 
salient/important  differences between the |> pipe and the %>% magrittr 
pipe, it would be great to have those documented (I know that 
documenting the difference between a base-R operator and the one that's 
implemented in a non-Recommended package is a little weird, but it would 
be helpful in this case ...)  I know I could go back to the mailing list 
discussion at 
https://hypatia.math.ethz.ch/pipermail/r-devel/2020-December/080173.html 
and try to figure it out for myself ...

   cheers
    Ben Bolker