Add '# Safety' section to 'unsafe fn' doc

src/board/color.rs

@@ -20,6 +20,10 @@ impl Color {
     }
 
     /// Convert from a piece index into a [Color] type, no bounds checking.
+    ///
+    /// # Safety
+    ///
+    /// Should only be called with values that can be output by [Color::index()].
     #[inline(always)]
     pub unsafe fn from_index_unchecked(index: usize) -> Self {
         std::mem::transmute(index as u8)