/usr/bin/rdoc Files are parsed, and the information they contain collected, before any output is produced. This allows cross references between all files to be resolved. If a name is a directory, it is traversed. If no names are specified, all Ruby files in the current directory (and subdirectories) are processed.
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/rdoc RDoc produces HTML and command-line documentation for Ruby projects. RDoc includes the rdoc tool for generating and displaying documentation from the command-line.
The binary is a part of Ruby public framework.