Refactor: scheduler: best practices for summary_action_flags()

Rename summary_action_flags() to pcmk__collective_action_flags().
The name is less than ideal since groups are also collective resources,
but instance_action_flags wouldn't be correct, and group_or_bundle_action_flags
seems long.

Also, declare it in the library-private header, add a doxygen block,
and improve variable names, comments, and formatting.