api: missing webdav chunked upload api documentation

[DeepDiver1975]

WHAT Needs to be Documented?

We invented this protocol extension - but the fork is getting the credits

rclone/rclone#6133

WHERE Does This Need To Be Documented (Link)?

https://docs.nextcloud.com/server/24/developer_manual/client_apis/WebDAV/chunking.html#chunked-file-upload

WHY Should This Change Be Made?

(Optional) What Type Of Content Change Is This?

• New Content Addition
• Old Content Deprecation
• Existing Content Simplification
• Bug Fix to Existing Content

(Optional) Which Manual Does This Relate To?

• Admin Manual
• Developer Manual
• User Manual
• Android
• iOS
• Branded Clients
• Desktop Client
• Other