docs: comment on volatile access to 0 in fn docs

Author: LuigiPiucco

library/core/src/ptr/mod.rs

@@ -1749,7 +1749,9 @@ pub const unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
 ///
 /// Volatile operations are intended to act on I/O memory, and are guaranteed
 /// to not be elided or reordered by the compiler across other volatile
-/// operations.
+/// operations. In addition, note that bit-address 0 is legal and can be used
+/// when it's known to hold I/O memory, so using these operations on a null
+/// pointer won't panic.
 ///
 /// # Notes
 ///
@@ -1828,7 +1830,9 @@ pub unsafe fn read_volatile<T>(src: *const T) -> T {
 ///
 /// Volatile operations are intended to act on I/O memory, and are guaranteed
 /// to not be elided or reordered by the compiler across other volatile
-/// operations.
+/// operations. In addition, note that bit-address 0 is legal and can be used
+/// when it's known to hold I/O memory, so using these operations on a null
+/// pointer won't panic.
 ///
 /// `write_volatile` does not drop the contents of `dst`. This is safe, but it
 /// could leak allocations or resources, so care should be taken not to overwrite