Merge branch 'docs/download-2fa-codes' into 'master'

Documentation: Add mention that recovery codes are downloadable.

See merge request gitlab-org/gitlab-ce!22483

doc/user/profile/account/two_factor_authentication.md

@@ -83,9 +83,11 @@ Click on **Register U2F Device** to complete the process.
 Recovery codes are not generated for U2F devices.
 
 Should you ever lose access to your one time password authenticator, you can use one of the ten provided
-backup codes to login to your account. We suggest copying or printing them for
-storage in a safe place. **Each code can be used only once** to log in to your
-account.
+backup codes to login to your account. We suggest copying them, printing them, or downloading them using
+the **Download codes** button for storage in a safe place.
+
+CAUTION: **Caution:**
+Each code can be used only once to log in to your account.
 
 If you lose the recovery codes or just want to generate new ones, you can do so
 [using SSH](#generate-new-recovery-codes-using-ssh).