This is really an addendum to my original post, to the effect that I have (in some sense) solved the problem. I also asked about the problem on the R-help mailing list and got a useful reply to the effect that Rstudio requires its own purpose-specific binaries.
I was always under the impression that Rstudio would run whatever instance of R that the user had installed, but this seems not to be the case. My respondent on R-help pointed me at instructions for installing R in such a way as to satisfy Rstudio.
I had seen no such instructions elsewhere.
After considerable travail I managed to effect this installation,
which put R into /opt/R/4.1.0 and lo and behold
/opt/R/4.1.0/lib/R does indeed contain a "doc" directory
(unlike, e.g. /usr/lib/R which is my non-Rstudio instance of R lives.)
Having done that and having made the appropriate symbolic links, I was able to click on the Rstudio icon under
Applications -> Programming and get Rstudio running.
So far I can find no way to get Rstudio to do what I had hoped to be able to do --- something that cannot effectively be done in raw R. But that's another story.
Let me end by saying that I was disappointed by the deafening silence with which my original post was met. Were it not for the R-help mailling list I would still be completely in the dark.