chore(metrics): document rate_limited reason for passkey create error
Because: * The passkey_create_submit_frontend_error Glean metric now emits a new "rate_limited" value for the "reason" extra-key, but the description in fxa-ui-metrics.yaml still listed only the prior values. Data reviewers and dashboards that key off this description should reflect the full set of values. This commit: * Adds "rate_limited" to the documented reason values for passkey_create_submit_frontend_error. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>