[libc][math][c23] Add sinf16 C23 math function

libc/src/math/generic/sinf16.cpp

@@ -62,20 +62,21 @@ LLVM_LIBC_FUNCTION(float16, sinf16, (float16 x)) {
       return r.value();
   }
 
-  // Exhaustive tests show that for |x| <= 0x13d0, 1ULP rounding errors occur.
-  // To fix this, the following apply:
-  // - When x >= 0, and rounding upward, sin(x) == x.
-  // - When x < 0, and rounding downward, sin(x) == x.
-  // - When x < 0, and rounding upward, sin(x) == (x - 1ULP)
   int rounding = fputil::quick_get_round();
+
+  // Exhaustive tests show that for |x| <= 0x1.f4p-11, 1ULP rounding errors
+  // occur. To fix this, the following apply:
   if (LIBC_UNLIKELY(x_abs <= 0x13d0)) {
+    // When x >= 0, and rounding upward, sin(x) == x.
     if (LIBC_UNLIKELY(x_abs == 0U))
       return x;
 
+    // When x < 0, and rounding downward, sin(x) == x.
     if ((rounding == FE_UPWARD && xbits.is_pos()) ||
         (rounding == FE_DOWNWARD && xbits.is_neg()))
       return x;
 
+    // When x < 0, and rounding upward, sin(x) == (x - 1ULP)
     if (rounding == FE_UPWARD && xbits.is_neg()) {
       x_u--;
       return FPBits(x_u).get_val();