kunit: Fix obsolete name in documentation headers (func->action)

The kunit_add_action() and related functions named the kunit_action_t
parameter 'func' in early drafts, which was later renamed to 'action'
However, the doc comments were not properly updated.

Fix these to avoid confusion and 'make htmldocs' warnings.

Fixes: b9dce8a1 ("kunit: Add kunit_add_action() to defer a call until test exit")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/lkml/20230530151840.16a56460@canb.auug.org.au/


Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>