I would expect them to be documented here: http://tutor.rascal-mpl.org/Rascal/Libraries/lang/java/m3/Core/Core.html, since that's the package you need to import to get them, but they're not there are searching the documentation for them doesn't result in much.
They are undocumented as of yet. Have a look at the (simple) code in the libraries. You can find them in the Rascal explorer as a library jar in lang/java/m3/Core.rsc, or online on github at https://github.com/cwi-swat/rascal/blob/master/src/org/rascalmpl/library/lang/java/m3/Core.rsc