add documentation

app/scripts/lib/seed-phrase-verifier.js

@@ -2,6 +2,11 @@ const KeyringController = require('eth-keyring-controller')
 
 const seedPhraseVerifier = {
 
+  // Verifies if the seed words can restore the accounts.
+  //
+  // The seed words can recreate the primary keyring and the accounts belonging to it.
+  // The created accounts in the primary keyring are always the same.
+  // The keyring always creates the accounts in the same sequence.
   verifyAccounts (createdAccounts, seedWords) {
 
     return new Promise((resolve, reject) => {

app/scripts/metamask-controller.js

@@ -600,6 +600,10 @@ module.exports = class MetamaskController extends EventEmitter {
     })
   }
 
+  // Verifies the current vault's seed words if they can restore the
+  // accounts belonging to the current vault.
+  //
+  // Called when the first account is created and on unlocking the vault.
   verifySeedPhrase (cb) {
 
     const primaryKeyring = this.keyringController.getKeyringsByType('HD Key Tree')[0]