doc: -f is now portable for cp and ln

doc: -f is now portable for cp and ln

This follows up on a comment by Glenn Morris in:
http://lists.gnu.org/archive/html/emacs-devel/2015-01/msg00011.html
* doc/autoconf.texi (Limitations of Usual Tools):
Remove circa-1990 advice about avoiding cp -f and ln -f.
Although that advice was reasonable for the early 1990s, the -f
option is portable for both cp and ln nowadays.  These options
were standardized in POSIX 1003.2-1992, and pre-POSIX systems such
as SunOS 4 are no longer of practical concern.