mx_fastr_dists: add doc comment

mx.fastr/mx_fastr_dists.py

@@ -264,6 +264,13 @@ class ReleaseBuildTask(mx.NativeBuildTask):
         self._template(rscript_launcher, join(bin_dir, 'Rscript'), template_dict)
 
 class FastRNativeRecommendedProject(mx.NativeProject):
+    '''
+    This finesses an ordering problem on installing the recommended R packages.
+    These must be installed by FastR using bin/R CMD INSTALL. That will invoke a
+    nested 'mx R' invocation which requires the FASTR distribution to be available.
+    However, this dependency cannt be specified in the suite.py file so we achieve
+    it here by ensuring that it is built prior to the native.recommended project.
+    '''
     def __init__(self, suite, name, deps, workingSets, theLicense, **args):
         mx.NativeProject.__init__(self, suite, name, None, [], deps, workingSets, None, None, join(suite.dir, name), theLicense)