Document v-model.number behavior for invalid strings (#1742)

Author: smith153

src/v2/api/index.md

@@ -2140,7 +2140,7 @@ type: api
 
 - **Modifiers:**
   - [`.lazy`](../guide/forms.html#lazy) - listen to `change` events instead of `input`
-  - [`.number`](../guide/forms.html#number) - cast input string to numbers
+  - [`.number`](../guide/forms.html#number) - cast valid input string to numbers
   - [`.trim`](../guide/forms.html#trim) - trim input
 
 - **Usage:**

src/v2/guide/forms.md

@@ -368,7 +368,7 @@ If you want user input to be automatically typecast as a number, you can add the
 <input v-model.number="age" type="number">
 ```
 
-This is often useful, because even with `type="number"`, the value of HTML input elements always returns a string.
+This is often useful, because even with `type="number"`, the value of HTML input elements always returns a string. If the value cannot be parsed with `parseFloat()`, then the original value is returned.
 
 ### `.trim`