This has been going on for a while, and I'm finally annoyed enough to post.
I've been using --no-rdoc --no-ri settings in a .gemrc file with rbenv-installed Ruby versions for many years, and this has always worked to suppress the addition of docs during gem installs.
But some time ago this stopped working, despite no changes to basic configurations or methods of installing new gems or ruby versions. So differences among these has had no effect upon the problem.
gem env shows configuration settings of "--no-rdoc --no-ri" for both "install" and "update," so RubyGems sees the settings.
Any idea what's going on? Some change to RubyGems or other? (Typically someone else would have posted, but searching multiple times hasn't turned up a recent surge of questions related to this.)
[Please note that this is not related to other Ruby version managers, or system-installed Ruby on MacOS.]
I think the answer may be found here.
My .gemrc was using --no-ri and --no-rdoc and while I read that these were deprecated, they may no longer be supported at all. Using --no-document instead seems to have solved the problem.