update multipart docs to mention ParamPart class

[technoweenie]

This updates the multipart documentation on the site. It was rewritten to include ParamsPart from #1017, while providing other important details and example usage.

• Await final class names for Faraday::UploadIO and Faraday::ParamsPart before merging

[iMacTia]

Clear and detailed 👍

[jeremyisr]

Hi @technoweenie, I see you updated the documentation. Were you expecting me to add an alias to ::UploadIO from ::Faraday::FilePart ?

[technoweenie]

@jeremy-israel Not at all! I started on the docs so I could see the new class names in action.

[technoweenie]

I updated this to use the latest class names from #1017 and #1021.

[olleolleolle]

This explains the new name for the renamed class, and also includes when ParamPart was introduced.

[olleolleolle]

Things documentation update brings clarity about the current (better) names.

[technoweenie]

Thanks for approving this so much! I'll try to use Draft PRs in the future :)

[olleolleolle]

Vigorous and enthusiastic approval is the best approval!