docs: Document @option.parameter_name

[NeloBlivion]

Summary

@discord.option.parameter_name is a kwarg that allows you to have separate parameter and UI names for an option.
E.g.

@slash_command()
async def say(ctx, file: Option(Attachment, name="image"))

would look like

@slash_command()
@option("image", parameter_name="file")
async def say(ctx, file: Attachment)

It seems to have existed since 2.1, but was never documented.

Information

• 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, typehinting,
  examples, ...).

Checklist

• I have searched the open pull requests for duplicates.
• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why.
• I have updated the changelog to include these changes.

[pullapprove4]

Please add a changelog entry