docs(combobox,command): cite Slint 1.16 string-method gap behind the v1 filter

I tried adding in-component substring filtering (visible + height collapse
pattern, avoiding the `for x: if c: Element` syntax that doesn't compile in
Slint 1.16). The blocker is the Slint string surface itself: `.to-lowercase()`
works, but `.contains()` and `.starts-with()` are not present in 1.16 (only
`is-empty` / `character-count` / `to-int` / `to-float` / case methods). Without
a substring primitive there's no pure-Slint substring filter.

Update both component comments so future readers (and a possible Slint
upgrade pass) know exactly what unblocks the inline filter.

Author: stevekwon211

registry/default/components/combobox.slint

@@ -73,8 +73,11 @@ export component Combobox inherits Rectangle {
                     edited(_) => { root.highlighted-index = 0; }
                 }
 
-                // Rows — derive `items` from `query` in the consumer; this
-                // component just shows whatever you pass.
+                // Rows — derive `items` from `query` in the consumer; Slint
+                // 1.16 has no substring/prefix string methods, so in-component
+                // filtering is blocked until Slint adds `contains` / `starts-
+                // with`. The component otherwise wires the search field + the
+                // selection + the keyboard contract cleanly.
                 for item[idx] in root.items: Rectangle {
                     height: 30px;
                     border-radius: Tokens.radius-sm;

registry/default/components/command.slint

@@ -76,8 +76,9 @@ export component Command inherits Rectangle {
                 edited(_) => { root.highlighted-index = 0; }
             }
 
-            // Rows — derive `items` from `query` in the consumer; this
-            // component just shows whatever you pass.
+            // Rows — derive `items` from `query` in the consumer; Slint 1.16
+            // has no substring/prefix string methods, so in-component filtering
+            // is blocked until Slint adds `contains` / `starts-with`.
             for item[i] in root.items: Rectangle {
                 height: 38px;
                 border-radius: Tokens.radius-sm;