Update docstring for protocol extension to mention Object

Description

Per https://ask.clojure.org/index.php/9950/document-extend-provide-default-protocol-implementation

The docstrings for extend, extend-type, and extend-protocol should mention extending to Object to act as a default implementation (this is mentioned on the reference page https://clojure.org/reference/protocols). Extending to nil is mentioned in `extend` docstring, but not Object.

It might also be helpful for `extend-type` and `extend-protocol` docstrings to refer to `extend` for more info.

Environment

None

Assignee

Unassigned

Reporter

Alex Miller

Labels

Approval

Triaged

Patch

None

Affects versions

Priority

Minor