docs(TouchID): Improves formatting of usage docs (#397)

* Fixes syntax error in usage example, and improves usage formatting

* Moves error codes into usage section

src/plugins/touchid.ts

@@ -8,6 +8,29 @@ import { Cordova, Plugin } from './plugin';
  *
  * Requires Cordova plugin: `cordova-plugin-touch-id`. For more info, please see the [TouchID plugin docs](https://github.com/EddyVerbruggen/cordova-plugin-touch-id).
  *
+ * @usage
+ * ### Import Touch ID Plugin into Project
+ * ```typescript
+ * import { TouchID } from 'ionic-native';
+ * ```
+ * ### Check for Touch ID Availability
+ * ```typescript
+ * TouchID.isAvailable()
+ *   .then(
+ *     res => console.log('TouchID is available!'),
+ *     err => console.error('TouchID is not available', err)
+ *   );
+ * ```
+ * ### Invoke Touch ID w/ Custom Message
+ *
+ * ```typescript
+ * TouchID.verifyFingerprint('Scan your fingerprint please')
+ *   .then(
+ *     res => console.log('Ok', res),
+ *     err => console.error('Error', err)
+ *   );
+ * ```
+ *
  * ### Error Codes
  *
  * The plugin will reject for various reasons. Your app will most likely need to respond to the cases differently.
@@ -20,23 +43,6 @@ import { Cordova, Plugin } from './plugin';
  *  -  `-4` - The scan was cancelled by the system (Home button for example)
  *  -  `-6` - TouchID is not Available
  *  -  `-8` - TouchID is locked out from too many tries
- * @usage
- * ```typescript
- * import { TouchID } from 'ionic-native';
- *
- *
- * TouchID.isAvailable()
- *   .then(
- *     res => console.log('TouchID is available!'),
- *     err => console.error("TouchID isn't available", err)
- *   );
- *
- * TouchID.verifyFingerprint('Scan your fingerprint please')
- *   .then(
- *     res => console.log('Ok', res),
- *     err => console.error('Error', err)
- *   );
- * ```
  */
 @Plugin({
   plugin: 'cordova-plugin-touch-id',
@@ -47,7 +53,7 @@ import { Cordova, Plugin } from './plugin';
 export class TouchID {
 
   /**
-   * Whether TouchID is available or not.
+   * Checks Whether TouchID is available or not.
    *
    * @return {Promise} Returns a Promise that resolves if yes, rejects if no.
    */