ipaclient_get_facts: Fix documentation sections and agument spec

ansible-test with ansible-2.14 is adding a lot of new tests to ensure
that the documentation section and the agument spec is complete. Needed
changes:

A copyright header needs to be present.

DOCUMENTATION section

- `author` needs to be given with the github user also: `Name (@user)`

Example section needs to be present, even if empty.

RETURN section needs to be present if parameters are returned.