[Rd] Proposed patch for ?Extract

[Martin Maechler]

>>>>> Marc Schwartz via R-devel 
>>>>>     on Thu, 21 Feb 2019 14:39:45 +0100 writes:

 >   Hi,

> In follow up to the thread on R-Help yesterday:
>    https://stat.ethz.ch/pipermail/r-help/2019-February/461725.html

> I am attaching a proposed patch against the trunk version of
> Extract.Rd, with wording added to the "Matrices and arrays" section, to
> note that indexing these object by factors behaves in a manner
> consistent with vectors.
> Regards,
> Marc Schwartz

Seems reasonable to me: some redundancy is allowed even in the
reference manuals (:= union of all help pages).
{good useRs knowning that 'matrix' and 'arrays' are atomic
 vectors, too .. as was mentioned.}

Martin