lavu/rational: add more info regarding floor(x+0.5) usage

Add some more verbose info regarding why the imprecise and slow floor(x+0.5) hack
is used; helpful for future maintenance.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanag@gmail.com>

libavutil/rational.c

@@ -115,7 +115,8 @@ AVRational av_d2q(double d, int max)
     frexp(d, &exponent);
     exponent = FFMAX(exponent-1, 0);
     den = 1LL << (61 - exponent);
-    // (int64_t)rint() and llrint() do not work with gcc on ia64 and sparc64
+    // (int64_t)rint() and llrint() do not work with gcc on ia64 and sparc64,
+    // see Ticket2713 for affected gcc/glibc versions
     av_reduce(&a.num, &a.den, floor(d * den + 0.5), den, max);
     if ((!a.num || !a.den) && d && max>0 && max<INT_MAX)
         av_reduce(&a.num, &a.den, floor(d * den + 0.5), den, INT_MAX);