Remove multi-swap extension (OR modify documentation) since by now (almost) everything can be done with hx-select-oob

[oliverhaas]

See my comment here bigskysoftware/htmx#1512 (comment).

Since 1.0.2 the multi-swap extensions became redundant, since hx-swap-oob and hx-select-oob started supporting nested swaps (see https://github.com/bigskysoftware/htmx/blob/master/CHANGELOG.md#102---2020-12-12).

This means the documentation of multi-swap (which says oob swaps can't handle nested swaps), as well as the actual existence of the extension itself, is misleading. I certainly used multi swap when I did not need to.

If there is anything I'm missing, let me know. Otherwise I'm open to making a PR.

[oliverhaas]

I've actually found reasons what multi-swap can do that hx-select-oob can't:

Since there is not Hx-Reselect-Oob response header, one has to use mutli-swap if one wants that functionality with response headers.

Same goes for htmx.ajax: Since there is not selectOobOverride (yet?) or similar, multi-swap here has some niche where one might want to use it.

So I would probably reduce the scope of this issue to modifying the docs so it's reflecting the information given here.