[llvm][docs] Expand section on non-integral pointers

Specify their restrictions w.r.t. `align` attribute.

Author: danilaml

llvm/docs/LangRef.rst

@@ -634,6 +634,12 @@ implementation defined, the optimizer can't do the latter.  The former is
 challenging as many commonly expected properties, such as
 ``ptrtoint(v)-ptrtoint(v) == 0``, don't hold for non-integral types.
 
+Similar restrictions apply to intrinsics that might examine the pointer bits,
+such as :ref:`llvm.ptrmask<int_ptrmask>`. In particular, it's not
+correct to infer `align` attributes for such pointers based on examining their
+bits and frontends must ensure that all `align` attributes represent the
+conservative alignment valid for every possible representation of the pointer.
+
 .. _globalvars:
 
 Global Variables