Fix poll parameter defaulting to MISSING instead of None for Context.send

[Soheab]

Summary

This is done purely for consistency with all the other parameters. Context: https://discord.com/channels/336642139381301249/336642776609456130/1353467110537298102

Checklist

• [ ] If code changes were made then they have been tested.
  • [x] I have updated the documentation to reflect the changes.
• [ ] This PR fixes an issue.
• [ ] This PR adds something new (e.g. new method or parameters).
• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [ ] This PR is not a code change (e.g. documentation, README, ...)