Fix word choice and pointer fragment rationale

Let's:

- Replace "complete, valid pointers" with "complete pointers".  The
  restriction is about byte provenance forming whole pointers, not
  about those pointers being dereferenceable or in-bounds.

- Replace the incorrect claim that "the compiler cannot support
  pointer fragments because it would be unable to reason about them
  at compile time" with the accurate explanation: binary formats
  such as ELF cannot represent pointer fragments, so the compiler
  cannot emit them in the final binary.

- Replace "on 64-bit platforms" with "assuming an 8-byte pointer"
  to make the platform assumption explicit without implying the
  example only works on 64-bit platforms.

Author: traviscross

src/const_eval.md

@@ -261,9 +261,9 @@ const C: Pair = unsafe {
 ```
 
 > [!NOTE]
-> The bytes with provenance must form a complete pointer in the correct order.  In the example above, the pointer is written at offset 20, but it requires (on 64-bit platforms) 8 bytes.  Four of those bytes fit in the `y` field; the rest extend into the padding at offset 24.  When the fields are initialized, the `y` bytes get overwritten, leaving only a partial pointer (4 bytes) in the padding.  These 4 bytes have provenance but don't form a complete pointer, causing compilation to fail.
+> The bytes with provenance must form a complete pointer in the correct order.  In the example above, the pointer is written at offset 20, but a pointer requires 8 bytes (assuming an 8-byte pointer).  Four of those bytes fit in the `y` field; the rest extend into the padding at offset 24.  When the fields are initialized, the `y` bytes get overwritten, leaving only a partial pointer (4 bytes) in the padding.  These 4 bytes have provenance but don't form a complete pointer, causing compilation to fail.
 >
-> This restriction ensures that any bytes with provenance in the final value represent complete, valid pointers.  The compiler cannot support pointer fragments because it would be unable to reason about them at compile time.
+> This restriction ensures that any bytes with provenance in the final value represent complete pointers.  Binary formats such as ELF cannot represent pointer fragments, so the compiler cannot emit them in the final binary.
 >
 > Reversing the order of the pointer bytes also causes compilation to fail, even though all bytes are present:
 >