fix: Add missing doc for Account Members Endpoints

Add documentation for:
* `GET /{account_slug}/members/{member_id}`
* `PUT /{account_slug}/members/{member_id}`
* `DELETE /{account_slug}/members/{member_id}`

Update documentation for:
* `POST /{account_slug}/members` - params should be in request body, not query params.

* I needed to `$ go get -u golang.org/x/sys` as `$ make generate` on
  macOS 12.6 with Go 1.19.3 (Homebrew) othwerwise failed with
  "go:linkname must refer to declared function or variable".
  The suggested solution to update sys comes from
  https://stackoverflow.com/a/71508032/265508

Fixes #395

Co-authored-by: Nicola Aitken <[email protected]>

Author: Erik Westrup