current() specification is too narrow

See this brilliant example by @lognaturel for a case where current() is used without any `instance()` involvement https://github.com/XLSForm/pyxform/pull/472#discussion_r494667102

Read the current() specification and the linked pyxform discussion, especially the example showing use without instance(). Determine which parts of the specification are too narrow and agree on the intended scope with maintainers. Done means the specification clearly covers the accepted use case and any related examples or tests are updated.

Written by the indexing model from the issue text.