It is project specific, although there are some standard locations. Look in the makefile for the project and track down the make command for the man pages or whatever documentation the project provides, and there you'll find what you seek.
Some projects separate it out, ejabbedctl is part of ejabberd and the docs for ejabberd are on a separate github repository, specifically the folder containing ejabberdcrl is content/admin/ejabberdctl/ in the docs.ejabberd repository.
For virtualenv, the project is python so they're using sphinx, which pulls a lot of the documentation straight from the source (much like doxygen, javadoc, et al) and converts sphinx code into several formats (html, latex, man, text, et al). If you look at it in github, it looks like it's the finished product because github knows how to display .rst files. The docs directory in the virtualenv source has things like the userguide, etc. You may find what you're looking for there. Since it could be embedded anywhere, I'd recommend pulling down the source and using grep to find the erroneous text, it could be literally anywhere.