doc: update unsafe.Pointer rule in spec

The valid conversions consider the core types of operands, not just
their underlying type.

This also explains the valid arguments for unsafe.Slice which are
explained in terms of unsafe.Pointer conversions.

unsafe.SliceData simply refers to "slice argument" and we use
similar terminology elsewhere in the spec to denote values that
have a core type of slice (or any other type for that matter).
Leaving alone for now.

Fixes golang#64452.

Change-Id: I0eed3abbc0606f22358835e5d434f026fe0909c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/551379
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Bypass: Robert Griesemer <gri@google.com>

doc/go_spec.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of Nov 1, 2023",
+	"Subtitle": "Version of Dec 19, 2023",
 	"Path": "/ref/spec"
 }-->
 
@@ -8232,8 +8232,8 @@ <h3 id="Package_unsafe">Package <code>unsafe</code></h3>
 <p>
 A <code>Pointer</code> is a <a href="#Pointer_types">pointer type</a> but a <code>Pointer</code>
 value may not be <a href="#Address_operators">dereferenced</a>.
-Any pointer or value of <a href="#Underlying_types">underlying type</a> <code>uintptr</code> can be
-<a href="#Conversions">converted</a> to a type of underlying type <code>Pointer</code> and vice versa.
+Any pointer or value of <a href="#Core_types">core type</a> <code>uintptr</code> can be
+<a href="#Conversions">converted</a> to a type of core type <code>Pointer</code> and vice versa.
 The effect of converting between <code>Pointer</code> and <code>uintptr</code> is implementation-defined.
 </p>
 
@@ -8244,6 +8244,10 @@ <h3 id="Package_unsafe">Package <code>unsafe</code></h3>
 type ptr unsafe.Pointer
 bits = *(*uint64)(ptr(&amp;f))
 
+func f[P ~*B, B any](p P) uintptr {
+	return uintptr(unsafe.Pointer(p))
+}
+
 var p ptr = nil
 </pre>