docs: add manual voice ID entry instructions to legacy migration guide

Add note explaining how users can manually enter voice IDs for voices
not listed in the Vapi Dashboard dropdown by using the "Add Voice ID
Manually" option.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Vapi Tasker

fern/providers/voice/vapi-voices/legacy-migration.mdx

@@ -66,6 +66,8 @@ We've selected replacement voices from ElevenLabs using their voice matching too
 | Lily | Tanya - `7sAIIIXrQKek5xiLc1R6` | Melissa - `JH3fX8OSjg6sNdEtPjxr` | Ariel - `TPbSfDVirzpiSkim8gMw` |
 | Kylie | Chanelle - `t5ztDJA7pj9EyW9QIcJ2` | Aryannah - `vOJuCTA36jkG6q1OskrK` | Riley - `kXsOSDWolD7e9l1Z0sbH` |
 
+For voices that are not in the list of voices on the Vapi Dashboard, users will need to check the "Add Voice ID Manually" option and paste the Voice ID in the text box.
+
 ## FAQs
 
 <AccordionGroup>