spec: clarify prose for embedded struct fields

The spec says that an embedded field must be specified
as a type name (or a pointer to a type name). This is
explicit in the prose and the FieldDecl syntax.

However, the prose on promoted methods required a named
type (originally the term used for a "defined type").
Before the introduction of alias types, type names could
only refer to named/defined types, so the prose was ok.

With the introduction of alias types in Go 1.9, we
distinguished between defined types (i.e., types given
a name through a type declaration) and type aliases
(types given an alternative name), and retired the notion
of a named type since any type with a name (alias type
and defined type) could be considered a "named type".

To make things worse, with Go 1.18 we re-introduced the
notion of a named type which now includes predeclared
types, defined types, type parameters (and with that
type aliases denoting named types).

In the process some of the wording on method promotion
didn't get updated correctly. At attempt to fix this
was made with CL 406054, but while that CL's description
correctly explained the intent, the CL changed the prose
from "defined type" to "named type" (which had the new
meaning after Go 1.18), and thus did not fix the issue.

This CL fixes that fix by using the term "type name".
This makes the prose consistent for embedded types and
in turn clarifies that methods of embedded alias types
(defined or not) can be promoted, consistent with the
implementation.

While at it, also document that the type of an embedded
field cannot be a type parameter. This restriction has
been in place since the introduction of type parameters
with Go 1.18 and is enforced by the compiler.

Fixes #66540.
For #41687.

Change-Id: If9e6a03d7b84d24a3e6a5ceda1d46bda99bdf1f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/603958
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Bypass: Robert Griesemer <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
Reviewed-by: Axel Wagner <[email protected]>

Author: griesemer

doc/go_spec.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Language version go1.23 (June 13, 2024)",
+	"Subtitle": "Language version go1.23 (Aug 12, 2024)",
 	"Path": "/ref/spec"
 }-->
 
@@ -1086,7 +1086,7 @@ <h3 id="Struct_types">Struct types</h3>
 An embedded field must be specified as
 a type name <code>T</code> or as a pointer to a non-interface type name <code>*T</code>,
 and <code>T</code> itself may not be
-a pointer type. The unqualified type name acts as the field name.
+a pointer type or type parameter. The unqualified type name acts as the field name.
 </p>
 
 <pre>
@@ -1127,7 +1127,7 @@ <h3 id="Struct_types">Struct types</h3>
 </p>
 
 <p>
-Given a struct type <code>S</code> and a <a href="#Types">named type</a>
+Given a struct type <code>S</code> and a type name
 <code>T</code>, promoted methods are included in the method set of the struct as follows:
 </p>
 <ul>