willingc comment on r readme

tests/r/simple/README.rst

@@ -11,8 +11,8 @@ two files:
    Where 'YYYY', 'MM' and 'DD' refer to a specific
    date snapshot of https://mran.microsoft.com/timemachine
    from which libraries will be installed.
-*  An optional ``install.R`` file that will be executed at build time and can
-   be used for installing packages from both MRAN and GitHub.
+*  An optional ``install.R`` file that will be executed by an R installation
+   at build time. It can be used for installing packages from MRAN or GitHub.
 
 The presence of ``runtime.txt`` is enough to set up R, RStudio, and IRKernel. It
 uses the ``r-base`` package from the Ubuntu apt repositories to install