Refactor: scheduler: best practices for group_internal_constraints()

Rename group_internal_constraints() to pcmk__group_internal_constraints(),
declare in library-private header, add a doxygen block, assert for invalid
argument, and improve comments and formatting.

Also, a stop ordering is moved for readability. This theoretically could
have some obscure side effects if the relevant actions have not already been
created, but stops should have been created already, and no regression tests
change, so it is highly unlikely that it matters in this case.