Update documentation to note the weird panic message.

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

src/addr.rs

@@ -531,8 +531,10 @@ impl Sub<PhysAddr> for PhysAddr {
 
 /// Align address downwards.
 ///
-/// Returns the greatest x with alignment `align` so that x <= addr. The alignment must be
-///  a power of 2.
+/// Returns the greatest `x` with alignment `align` so that `x <= addr`.
+///
+/// Panics if the alignment is not a power of two. Without the `const_fn`
+/// feature, the panic message will be "index out of bounds".
 #[inline]
 pub const fn align_down(addr: u64, align: u64) -> u64 {
     const_assert!(align.is_power_of_two(), "`align` must be a power of two");
@@ -541,8 +543,10 @@ pub const fn align_down(addr: u64, align: u64) -> u64 {
 
 /// Align address upwards.
 ///
-/// Returns the smallest x with alignment `align` so that x >= addr. The alignment must be
-/// a power of 2.
+/// Returns the smallest `x` with alignment `align` so that `x >= addr`.
+///
+/// Panics if the alignment is not a power of two. Without the `const_fn`
+/// feature, the panic message will be "index out of bounds".
 #[inline]
 pub const fn align_up(addr: u64, align: u64) -> u64 {
     const_assert!(align.is_power_of_two(), "`align` must be a power of two");

src/structures/gdt.rs

@@ -141,7 +141,8 @@ impl GlobalDescriptorTable {
     const_fn! {
         /// Adds the given segment descriptor to the GDT, returning the segment selector.
         ///
-        /// Panics if the GDT has no free entries left.
+        /// Panics if the GDT has no free entries left.  Without the `const_fn`
+        /// feature, the panic message will be "index out of bounds".
         #[inline]
         pub fn add_entry(&mut self, entry: Descriptor) -> SegmentSelector {
             let index = match entry {