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
diff --git a/envsetup.sh b/envsetup.sh
index 5170063..c688331 100644
--- a/envsetup.sh
+++ b/envsetup.sh
@@ -7,6 +7,7 @@
 - m:       Makes from the top of the tree.
 - mm:      Builds all of the modules in the current directory, but not their dependencies.
 - mmm:     Builds all of the modules in the supplied directories, but not their dependencies.
+           To limit the modules being built use the syntax: mmm dir/:target1,target2.
 - mma:     Builds all of the modules in the current directory, and their dependencies.
 - mmma:    Builds all of the modules in the supplied directories, and their dependencies.
 - cgrep:   Greps on all local C/C++ files.