fix(docs): Improved QInput example...

...to prevent issue duplicating

Related: [quasarframework#11423, [quasarframework#10376, [quasarframework#10362 [quasarframework#12431.

docs/src/pages/vue-components/input.md

@@ -74,6 +74,10 @@ Please check these resources for more information about native attributes (for i
 
 As a helper, you can use `clearable` prop so user can reset model to `null` through an appended icon. The second QInput in the example below is the equivalent of using `clearable`.
 
+::: warning
+Won't work with `v-model`-managed input modifiers, such as `.trim`, because in that case `Vue` doesn't handle `null` values.
+:::
+
 <doc-example title="Clearable" file="QInput/Clearable" />
 
 ### Input types