explain what 'trim' does (#1912)

* explain what 'trim' does

'This trims out white-space from the input. '

* simplify language in v-model.trim explanation

Author: Katinka Hesselink

Diff for: src/v2/guide/forms.md

@@ -372,7 +372,7 @@ This is often useful, because even with `type="number"`, the value of HTML input
 
 ### `.trim`
 
-If you want user input to be trimmed automatically, you can add the `trim` modifier to your `v-model` managed inputs:
+If you want whitespace from user input to be trimmed automatically, you can add the `trim` modifier to your `v-model`-managed inputs:
 
 ```html
 <input v-model.trim="msg">