Fix issue 22417 - Slice assignment operator overloading example is incorrect

Also update the surrounding exposition to match the corrected example,
and change some identifiers in the example to match the notation used in
the exposition and the comments.

Author: pbackus

spec/operatoroverloading.dd

@@ -602,7 +602,7 @@ $(H3 $(LNAME2 slice_assignment_operator, Slice Assignment Operator Overloading))
         `opIndexAssign` member function that takes the return value of the
         `opSlice` function as parameter(s).
         Expressions of the form $(CODE a[)$(SLICE)$(D ] = c) are rewritten as
-        $(CODE a.opIndexAssign$(LPAREN)c,) $(D a.opSlice$(LPAREN))$(SLICE2)$(D $(RPAREN)$(RPAREN)),
+        $(CODE a.opIndexAssign$(LPAREN)c,) $(D a.opSlice!0$(LPAREN))$(SLICE2)$(D $(RPAREN)$(RPAREN)),
         and $(CODE a[] = c) as $(CODE a.opIndexAssign(c)).
         )
 
@@ -613,23 +613,23 @@ $(H3 $(LNAME2 slice_assignment_operator, Slice Assignment Operator Overloading))
 -------
 struct A
 {
-    int $(CODE_HIGHLIGHT opIndexAssign)(int v);  // overloads a[] = v
-    int $(CODE_HIGHLIGHT opIndexAssign)(int v, size_t[2] x);  // overloads a[i .. j] = v
-    int[2] $(CODE_HIGHLIGHT opSlice)(size_t x, size_t y);     // overloads i .. j
+    int opIndexAssign(int v);  // overloads a[] = v
+    int opIndexAssign(int v, size_t[2] slice);  // overloads a[i .. j] = v
+    size_t[2] opSlice(size_t dim)(size_t i, size_t j);  // overloads i .. j
 }
 
 void test()
 {
     A a;
     int v;
 
-    a$(CODE_HIGHLIGHT []) = v;  // same as a.opIndexAssign(v);
-    a$(CODE_HIGHLIGHT [)3..4$(CODE_HIGHLIGHT ]) = v;  // same as a.opIndexAssign(v, a.opSlice(3,4));
+    a[] = v;  // same as a.opIndexAssign(v);
+    a[3..4] = v;  // same as a.opIndexAssign(v, a.opSlice!0(3,4));
 }
 -------
 
         $(P For backward compatibility, if rewriting $(D a[)$(SLICE)$(D ]) as
-        $(D a.opIndexAssign$(LPAREN)a.opSlice$(LPAREN))$(SLICE2)$(D $(RPAREN)$(RPAREN))
+        $(D a.opIndexAssign$(LPAREN)a.opSlice!0$(LPAREN))$(SLICE2)$(D $(RPAREN)$(RPAREN))
         fails to compile, the legacy rewrite
         $(D opSliceAssign$(LPAREN)c,) $(SLICE2)$(D $(RPAREN)) is used instead.
         )