[ENH] Clarify rsync flavors (PR#2886)

The 1.7.0 (2003-04-16) "R Installation and Adminsistration"
manual mentions various flavors of R available from rsync (section
1.2, p. 1).  These are also referred to in various other sources on
and offline (e.g., the FAQ).

The meaning of r-release vs r-patched was not entirely clear to me.
How is the patched version patched?  Should it just have bug fixes,
and so likely be more reliable than r-release?  Or is it patched with
a lot of development changes, so that it's quite like r-devel?

It would be helpful to say a little more about these matters.