Updated the comment about the optimization for tiny x (the previous

commit moved it).  This includes a comment that the "kernel" sine no
longer works on arg -0, so callers must now handle this case.  The kernel
sine still works on all other tiny args; without the optimization it is
just a little slower on these args.  I intended it to keep working on
all tiny args, but that seems to be impossible without losing efficiency
or accuracy.  (sin(x) ~ x * (1 + S1*x**2 + ...) would preserve -0, but
the approximation must be written as x + S1*x**3 + ... for accuracy.)

lib/msun/src/k_sin.c

@@ -16,14 +16,16 @@ static char rcsid[] = "$FreeBSD$";
 #endif
 
 /* __kernel_sin( x, y, iy)
- * kernel sin function on [-pi/4, pi/4], pi/4 ~ 0.7854
+ * kernel sin function on ~[-pi/4, pi/4] (except on -0), pi/4 ~ 0.7854
  * Input x is assumed to be bounded by ~pi/4 in magnitude.
  * Input y is the tail of x.
  * Input iy indicates whether y is 0. (if iy=0, y assume to be 0). 
  *
  * Algorithm
  *	1. Since sin(-x) = -sin(x), we need only to consider positive x. 
- *	2. if x < 2^-27 (hx<0x3e400000 0), return x with inexact if x!=0.
+ *	2. Callers must return sin(-0) = -0 without calling here since our
+ *	   odd polynomial is not evaluated in a way that preserves -0.
+ *	   Callers may do the optimization sin(x) ~ x for tiny x.
  *	3. sin(x) is approximated by a polynomial of degree 13 on
  *	   [0,pi/4]
  *		  	         3            13