Follow-up #20347: incorporate review about _get_series_list
Closes #20922

recap of #20347 :
Enabling alignment on top of the (v.0.22-)legal list of lists was relatively complex. The allowed argument types implemented in #20347 are as follows:

```
Type of (element of) "others"        | alone | within list-like
---------------------------------------------------------------------
pd.Series                            |  yes  | yes
pd.Index                             |  yes  | yes
np.ndarray (1-dim)                   |  yes  | yes
DataFrame                            |  yes  | no
np.ndarray (2-dim)                   |  yes  | no
iterators, dict-views, etc.          |  yes  | **
anything else w/ is_list_like = True |  yes  | **
```

** to be permitted, list-likes (say `nxt`) within a list-like `others` must pass (essentially)
```all(not is_list_like(x) for x in nxt)```
and not be a DF.

Open review points from #20347 from @jreback (and my responses) reproduced in review comments.

PS. Managed to have a brainfart and there's a bug in the RC docs - very sorry. Fixed in this PR.