[LangRef] Mention that freeze does not consider aggregate's paddings

Make explicit that freeze does not touch paddings of an aggregate.
(Relevant comment: https://reviews.llvm.org/D83752#2152550)

This implies that `v = freeze(load p); store v, q` may still leave undef bits
or poison in memory if `v` is an aggregate, but it still happens for
non-byte integers such as i1.

Differential Revision: https://reviews.llvm.org/D83927

llvm/docs/LangRef.rst

@@ -3523,6 +3523,9 @@ uses with" concept would not hold.
 To ensure all uses of a given register observe the same value (even if
 '``undef``'), the :ref:`freeze instruction <i_freeze>` can be used.
 A value is frozen if its uses see the same value.
+An aggregate value or vector is frozen if its elements are frozen.
+The padding of an aggregate isn't considered, since it isn't visible
+without storing it into memory and loading it with a different type.
 
 .. code-block:: llvm
 
@@ -10682,6 +10685,9 @@ instructions may yield different values.
 While ``undef`` and ``poison`` pointers can be frozen, the result is a
 non-dereferenceable pointer. See the
 :ref:`Pointer Aliasing Rules <pointeraliasing>` section for more information.
+If an aggregate value or vector is frozen, the operand is frozen element-wise.
+The padding of an aggregate isn't considered, since it isn't visible
+without storing it into memory and loading it with a different type.
 
 
 Example: