GB079 10.01 Add example for private-module-fragment

[wg21bot]

The private-module-fragment is mentioned many times in the standard but its usage its obscure; as is in the papers. Nobody in the BSI knows its intended usage.
Please provide an example.

Proposed change:
Example added

[wg21bot]

See also US036 #35.

[davidstone]

SG2 recommends forwarding to CWG to clarify the current rules, not just by adding an example.

[jensmaurer]

CWG in Belfast: Tentatively accept; drafting assigned.

[jensmaurer]

CWG in Belfast: Accepted. See paper P1971R0.