[2FA] Add troubleshoot info when `invalid pin code` error occurs

Problem to solve

There are many issues created with same problem of two-factor authentication - invalid pin code. And most of them caused by time desync. It was described in troubleshoot section of docs, but it takes time for users to find it.

Proposal

Add this text or at least link to https://docs.gitlab.com/ce/user/profile/account/two_factor_authentication.html#troubleshooting

If you are receiving an invalid pin code error, this may indicate that there is a time sync issue between the authentication application and the GitLab instance itself.

Most authentication apps have a feature in the settings for syncing the time for the codes themselves. For Google Authenticator for example, go to Settings > Time correction for codes