Add usage comment to explain how to limit modules in mmm.

68895a96 has introduced the possibility to limit the modules being
built by mmm. In many occasions this can save a lot of developement
time since it allows to build one target without processing all the
makefiles in the Android tree. Unfortunatelly this nice feature is
undocumented. This CL adds the one line documentation.

Change-Id: I6ded32c4774047599e9c44e5ea71c07e65731cda