add tagalong argument in the docstr

[araichoor]

This PR adds the tagalong argument in the docstr of the functions using it in targets.py and assign.py.

This was raised in #429 (and the taglong was introduced in PR #353).

tagalong is also used in some vis.py functions, but I m not at all familiar with those, and they haven t any docstr yet, so I didn t do any modification there.

[dstndstn]

Could you maybe drop the "A new one can be created..." sentence except for the one on "load_target_file"? Because a newly-created one is passed to the load functions, then that same one has to be passed to the write functions.

[araichoor]

Sure, done.
Thanks for the check!

[forero]

Thanks @dstndstn @araichoor !