spec: more precise prose for special case of append

As written, the special case for append may be interpreted
such that any first argument that is assignable to []byte
is permissible, including nil. This change makes makes it
clear that a slice argument is required.

Not a language change. The compiler always expected a slice
argument.

Fixes #76226.

Change-Id: I73a1725d10096690335d6edf5793beb5de73578f
Reviewed-on: https://go-review.googlesource.com/c/go/+/718880
Reviewed-by: Junyang Shao <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Robert Griesemer <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: Robert Griesemer

doc/go_spec.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Language version go1.26 (Oct 1, 2025)",
+	"Subtitle": "Language version go1.26 (Nov 9, 2025)",
 	"Path": "/ref/spec"
 }-->
 
@@ -7383,8 +7383,8 @@ <h3 id="Appending_and_copying_slices">Appending to and copying slices</h3>
 where <code>E</code> is the element type of <code>S</code>
 and the respective <a href="#Passing_arguments_to_..._parameters">parameter
 passing rules</a> apply.
-As a special case, <code>append</code> also accepts a first argument assignable
-to type <code>[]byte</code> with a second argument of string type followed by
+As a special case, <code>append</code> also accepts a slice whose type is assignable to
+type <code>[]byte</code> with a second argument of <code>string</code> type followed by
 <code>...</code>.
 This form appends the bytes of the string.
 </p>