Mosaic is what we call a “Remote Module”, i.e. it is an application created and maintained by an external contributor (https://github.com/remicres/otb-mosaic). In particular it is an “official” remote module, which means it is packaged (it is available when you download otb binaries).
The documentation for remote modules is not in the Cookbook in OTB 6.6.1, I’m not exactly sure why… But this will change in OTB 6.7 (coming soon )
In the meantime you can use the Cookbook generated from the develop branch of otb, available here (generated the 07/08/2019).
You can also print the documentation in a terminal by using