Update description about .number modifier

[zxuqian]

Description of Problem

It seems Vue 3 can automatically convert a string value to a number from a number input, i.e. <input type="number"：

Vue Source Code：

const castToNumber = number || (vnode.props && vnode.props.type === 'number');

The documentation about .number modifier does not reflect this.

Proposed Solution

I made some changes to the docs, to make it clearer about this situation.

Additional Information

No.