[Document] add form components english document (#199)

* [Document] add english document of Checkbox

* [Document] add english document of Field

* [Document] add english document of NumberKeyboard

* [bugfix] NumberKeyboard should not dispaly title when title is empty

* [Document] add english document of PasswordInput

* [Document] add english document of Radio

* [document] add english document of Switch

* [bugfix] remove redundent styles in english document

* [Document] fix details

* fix Switch test cases

docs/examples-docs/en-US/password-input.md

@@ -0,0 +1,88 @@
+<script>
+export default {
+  data() {
+    return {
+      value: '',
+      showKeyboard: true
+    }
+  },
+
+  methods: {
+    onInput(key) {
+      this.value = (this.value + key).slice(0, 6);
+    },
+    onDelete() {
+      this.value = this.value.slice(0, this.value.length - 1);
+    }
+  }
+}
+</script>
+
+## PasswordInput
+The PasswordInput component is usually used with [NumberKeyboard](#/en-US/component/number-keyboard) Component.
+
+### Install
+``` javascript
+import { PasswordInput, NumberKeyBoard } from 'vant';
+
+Vue.component(PasswordInput.name, PasswordInput);
+Vue.component(NumberKeyBoard.name, NumberKeyBoard);
+```
+
+### Usage
+
+#### Basic Usage
+
+:::demo Basic Usage
+```html
+<!-- PasswordInput -->
+<van-password-input
+  :value="value"
+  info="Some tips"
+  @focus="showKeyboard = true"
+></van-password-input>
+
+<!-- NumberKeyboard -->
+<van-number-keyboard
+  :show="showKeyboard"
+  @input="onInput"
+  @delete="onDelete"
+  @blur="showKeyboard = false"
+></van-number-keyboard>
+```
+
+```javascript
+export default {
+  data() {
+    return {
+      value: '',
+      showKeyboard: true
+    }
+  },
+
+  methods: {
+    onInput(key) {
+      this.value = (this.value + key).slice(0, 6);
+    },
+    onDelete() {
+      this.value = this.value.slice(0, this.value.length - 1);
+    }
+  }
+}
+```
+:::
+
+### API
+
+| Attribute | Description | Type | Default | Accepted Values |
+|-----------|-----------|-----------|-------------|-------------|
+| value | Password value | `String`  | `''` | - |
+| length | Maxlength of password | `Number` | `6` | - |
+| info | Bottom info | `String` | - | - |
+| errorInfo | Bottom error info | `String` | - | - |
+
+### Event
+
+| Event | Description | Attribute |
+|-----------|-----------|-----------|
+| focus | Triggered when input get focused | - |