Update Rtools requirement.

git-svn-id: https://svn.r-project.org/R/trunk@87277 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: kalibera

doc/manual/rw-FAQ.texi

@@ -109,6 +109,11 @@ Windows 10 and Windows Server 2016 or newer. On earlier versions of Windows,
 @abbr{UCRT} has to be installed before installing R.  @abbr{UCRT} is available for Windows since
 Windows Vista SP2 and Windows Server 2008 SP2. 
 
+@I{Rtools44} as of update 6335 requires at least Windows 8.1.  This
+requirement comes from @I{Msys2}, so it might still be possible to use build
+tools from an older version of @I{Rtools} or @I{Msys2} with the current
+toolchain and libraries from @I{Rtools}.
+
 @c 2000 went end-of-life 2010-07-13
 @c XP SP3 went end of life 2014-04-08
 @c Vista went end of life 2017-04-11