Function spec docs needs to clarify the path annotation details

[droot]

This came up in a PR --> https://github.com/GoogleContainerTools/kpt/pull/1699#discussion_r608874780

Copying the content from the PR:

We need to fully specify the API here, it's part of the Functions Spec. Current docs:

https://github.com/kubernetes-sigs/kustomize/blob/master/cmd/config/docs/api-conventions/config-io.md

This is ambiguous:

When writing Resources to a directory, the Resource SHOULD be written to the corresponding path relative to that directory.

We need to define which directory this refers to.